Microprocessor with boot indicator that indicates a boot ISA of the microprocessor as either the X86 ISA or the ARM ISA

ABSTRACT

A microprocessor includes a plurality of registers that holds an architectural state of the microprocessor and an indicator that indicates a boot instruction set architecture (ISA) of the microprocessor as either the x86 ISA or the Advanced RISC Machines (ARM) ISA. The microprocessor also includes a hardware instruction translator that translates x86 ISA instructions and ARM ISA instructions into microinstructions. The hardware instruction translator translates, as instructions of the boot ISA, the initial ISA instructions that the microprocessor fetches from architectural memory space after receiving a reset signal. The microprocessor also includes an execution pipeline, coupled to the hardware instruction translator. The execution pipeline executes the microinstructions to generate results defined by the x86 ISA and ARM ISA instructions. In response to the reset signal, the microprocessor initializes its architectural state in the plurality of registers as defined by the boot ISA prior to fetching the initial ISA instructions.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. Non-Provisional patentapplication Ser. No. 13/224,310, filed Sep. 1, 2011, which claimspriority based on the following U.S. Provisional applications, each ofwhich is hereby incorporated by reference in its entirety.

Ser. No. Filing Date Title 61/473,062 Apr. 07, 2011 MICROPROCESSOR WITHCONDITIONAL LOAD INSTRUCTION 61/473,067 Apr. 07, 2011 APPARATUS ANDMETHOD FOR USING BRANCH PREDICTION TO EFFICIENTLY EXECUTE CONDITIONALNON-BRANCH INSTRUCTIONS 61/473,069 Apr. 07, 2011 APPARATUS AND METHODFOR HANDLING OF MODIFIED IMMEDIATE CONSTANT DURING TRANSLATION

FIELD OF THE INVENTION

The present invention relates in general to the field ofmicroprocessors, and particularly to support of multiple instruction setarchitectures thereon.

BACKGROUND OF THE INVENTION

The x86 processor architecture, originally developed by IntelCorporation of Santa Clara, Calif., and the Advanced RISC Machines (ARM)architecture, originally developed by ARM Ltd. of Cambridge, UK, arewell known in the art of computing. Many computing systems exist thatinclude an ARM or x86 processor, and the demand for them appears to beincreasing rapidly. Presently, the demand for ARM architectureprocessing cores appears to dominate low power, low cost segments of thecomputing market, such as cell phones, PDA's, tablet PCs, networkrouters and hubs, and set-top boxes (for example, the main processingpower of the Apple iPhone and iPad is supplied by an ARM architectureprocessor core), while the demand for x86 architecture processorsappears to dominate market segments that require higher performance thatjustifies higher cost, such as in laptops, desktops and servers.However, as the performance of ARM cores increases and the powerconsumption and cost of certain models of x86 processors decreases, theline between the different markets is evidently fading, and the twoarchitectures are beginning to compete head-to-head, for example inmobile computing markets such as smart cellular phones, and it is likelythey will begin to compete more frequently in the laptop, desktop andserver markets.

This situation may leave computing device manufacturers and consumers ina dilemma over which of the two architectures will predominate and, morespecifically, for which of the two architectures software developerswill develop more software. For example, some entities purchase verylarge amounts of computing systems each month or year. These entitiesare highly motivated to buy systems that are the same configuration dueto the cost efficiencies associated with purchasing large quantities ofthe same system and the simplification of system maintenance and repair,for example. However, the user population of these large entities mayhave diverse computing needs for these single configuration systems.More specifically, some of the users have computing needs in which theywant to run software on an ARM architecture processor, and some havecomputing needs in which they want to run software on an x86architecture processor, and some may even want to run software on both.Still further, new previously-unanticipated computing needs may emergethat demand one architecture or the other. In these situations, aportion of the extremely large investment made by these large entitiesmay have been wasted. For another example, a given user may have acrucial application that only runs on the x86 architecture so hepurchases an x86 architecture system, but a version of the applicationis subsequently developed for the ARM architecture that is superior tothe x86 version (or vice versa) and therefore the user would like toswitch. Unfortunately, he has already made the investment in thearchitecture that he does not prefer. Still further, a given user mayhave invested in applications that only run on the ARM architecture, butthe user would also like to take advantage of fact that applications inother areas have been developed for the x86 architecture that do notexist for the ARM architecture or that are superior to comparablesoftware developed for the ARM architecture, or vice versa. It should benoted that although the investment made by a small entity or anindividual user may not be as great as by the large entity in terms ofmagnitude, nevertheless in relative terms the investment wasted may beeven larger. Many other similar examples of wasted investment may existor arise in the context of a switch in dominance from the x86architecture to the ARM architecture, or vice versa, in variouscomputing device markets. Finally, computing device manufacturers, suchas OEMs, invest large amounts of resources into developing new products.They are caught in the dilemma also and may waste some of their valuabledevelopment resources if they develop and manufacture mass quantities ofa system around the x86 or ARM architecture and then the user demandchanges relatively suddenly.

It would be beneficial for manufacturers and consumers of computingdevices to be able to preserve their investment regardless of which ofthe two architectures prevails. Therefore, what is needed is a solutionthat would allow system manufacturers to develop computing devices thatenable users to run both x86 architecture and ARM architecture programs.

The desire to have a system that is capable of running programs of morethan one instruction set has long existed, primarily because customersmay make a significant investment in software that runs on old hardwarewhose instruction set is different from that of the new hardware. Forexample, the IBM System/360 Model 30 included an IBM System 1401compatibility feature to ease the pain of conversion to the higherperformance and feature-enhanced System/360. The Model 30 included botha System/360 and a 1401 Read Only Storage (ROS) Control, which gave itthe capability of being used in 1401 mode if the Auxiliary Storage wasloaded with needed information beforehand. Furthermore, where thesoftware was developed in a high-level language, the new hardwaredeveloper may have little or no control over the software compiled forthe old hardware, and the software developer may not have a motivationto re-compile the source code for the new hardware, particularly if thesoftware developer and the hardware developer are not the same entity.Silberman and Ebcioglu proposed techniques for improving performance ofexisting (“base”) CISC architecture (e.g., IBM S/390) software byrunning it on RISC, superscalar, and Very Long Instruction Word (VLIW)architecture (“native”) systems by including a native engine thatexecutes native code and a migrant engine that executes base objectcode, with the ability to switch between the code types as necessarydepending upon the effectiveness of translation software that translatesthe base object code into native code. See “An Architectural Frameworkfor Supporting Heterogeneous Instruction-Set Architectures,” Sibermanand Ebcioglu, Computer, June 1993, No. 6. Van Dyke et al. disclosed aprocessor having an execution pipeline that executes native RISC(Tapestry) program instructions and which also translates x86 programinstructions into the native RISC instructions through a combination ofhardware translation and software translation, in U.S. Pat. No.7,047,394, issued May 16, 2006. Nakada et al. proposed a heterogeneousSMT processor with an Advanced RISC Machines (ARM) architecturefront-end pipeline for irregular (e.g., OS) programs and a Fujitsu FR-V(VLIW) architecture front-end pipeline for multimedia applications thatfeed an FR-V VLIW back-end pipeline with an added VLIW queue to holdinstructions from the front-end pipelines. See “OROCHI: A MultipleInstruction Set SMT Processor,” Proceedings of the First InternationalWorkshop on New Frontiers in High-performance and Hardware-awareComputing (HipHaC'08), Lake Como, Italy, November 2008 (In conjunctionwith MICRO-41), Buchty and Weib, eds, Universitatsverlag Karlsruhe, ISBN978-3-86644-298-6. This approach was proposed in order to reduce thetotal system footprint over heterogeneous System on Chip (SOC) devices,such as the Texas Instruments OMAP that includes an ARM processor coreplus one or more co-processors (such as the TMS320, various digitalsignal processors, or various GPUs) that do not share instructionexecution resources but are instead essentially distinct processingcores integrated onto a single chip.

Software translators, also referred to as software emulators, softwaresimulators, dynamic binary translators and the like, have also beenemployed to support the ability to run programs of one architecture on aprocessor of a different architecture. A popular commercial example isthe Motorola 68K-to-PowerPC emulator that accompanied Apple Macintoshcomputers to permit 68K programs to run on a Macintosh with a PowerPCprocessor, and a PowerPC-to-x86 emulator was later developed to permitPowerPC programs to run on a Macintosh with an x86 processor. TransmetaCorporation of Santa Clara, Calif., coupled VLIW core hardware and “apure software-based instruction translator [referred to as “CodeMorphing Software”] [that] dynamically compiles or emulates x86 codesequences” to execute x86 code. “Transmeta.” Wikipedia. 2011. WikimediaFoundation, Inc. en.wikipedia.org/wiki/Transmeta. See also, for example,U.S. Pat. No. 5,832,205, issued Nov. 3, 1998 to Kelly et al. The IBMDAISY (Dynamically Architected Instruction Set from Yorktown) systemincludes a VLIW machine and dynamic binary software translation toprovide 100% software compatible emulation of old architectures. DAISYincludes a Virtual Machine Monitor residing in ROM that parallelizes andsaves the VLIW primitives to a portion of main memory not visible to theold architecture in hopes of avoiding re-translation on subsequentinstances of the same old architecture code fragments. DAISY includesfast compiler optimization algorithms to increase performance. QEMU is amachine emulator that includes a software dynamic translator. QEMUemulates a number of CPUs (e.g., x86, PowerPC, ARM and SPARC) on varioushosts (e.g., x86, PowerPC, ARM, SPARC, Alpha and MIPS). As stated by itsoriginator, the “dynamic translator performs a runtime conversion of thetarget CPU instructions into the host instruction set. The resultingbinary code is stored in a translation cache so that it can be reused .. . . QEMU is much simpler [than other dynamic translators] because itjust concatenates pieces of machine code generated off line by the GNU CCompiler.” QEMU, a Fast and Portable Dynamic Translator, FabriceBellard, USENIX Association, FREENIX Track: 2005 USENIX Annual TechnicalConference. See also, “ARM Instruction Set Simulation on Multi-Core x86Hardware,” Lee Wang Hao, thesis, University of Adelaide, Jun. 19, 2009.However, while software translator-based solutions may providesufficient performance for a subset of computing needs, they areunlikely to provide the performance required by many users.

Static binary translation is another technique that has the potentialfor high performance. However, there are technical considerations (e.g.,self-modifying code, indirect branches whose value is known only atrun-time) and commercial/legal barriers (e.g., may require the hardwaredeveloper to develop channels for distribution of the new programs;potential license or copyright violations with the original programdistributors) associated with static binary translation.

BRIEF SUMMARY OF INVENTION

Embodiments of the present invention are described herein that addressthe needs identified above by providing a single processor design thatis capable of running x86 instruction set architecture (ISA) machinelanguage programs and ARM ISA machine language programs.

In one aspect, the present invention provides a microprocessor. Themicroprocessor includes a plurality of registers that holds anarchitectural state of the microprocessor. The microprocessor alsoincludes an indicator that indicates a boot instruction set architecture(ISA) of the microprocessor as either the x86 ISA or the Advanced RISCMachines (ARM) ISA. The microprocessor also includes a hardwareinstruction translator that translates x86 ISA instructions and ARM ISAinstructions into microinstructions. The hardware instruction translatortranslates, as instructions of the boot ISA, the initial ISAinstructions that the microprocessor fetches from architectural memoryspace after receiving a reset signal. The microprocessor also includesan execution pipeline, coupled to the hardware instruction translator.The execution pipeline executes the microinstructions to generateresults defined by the x86 ISA and ARM ISA instructions. In response tothe reset signal, the microprocessor initializes its architectural statein the plurality of registers as defined by the boot ISA prior tofetching the initial ISA instructions.

In another aspect, the present invention provides a method. The methodincludes detecting that a reset of a microprocessor has been signaled.The detecting is performed by the microprocessor. The method alsoincludes determining, in response to said detecting that a reset of themicroprocessor has been signaled, which of the x86 instruction setarchitecture (ISA) and the Advanced RISC Machines (ARM) ISA an indicatorof the microprocessor indicates is a boot ISA of the microprocessor. Thedetermining is performed by the microprocessor. The method also includesinitializing an architectural state of the microprocessor as defined bythe boot ISA. The initializing is performed by the microprocessor. Themethod also includes translating into microinstructions the initial ISAinstructions fetched from architectural memory space by themicroprocessor after the reset as instructions of the boot ISA. Thetranslating is performed by a hardware instruction translator of themicroprocessor. The method also includes executing the microinstructionsto generate results defined by the boot ISA. The executing is performedby an execution pipeline of the microprocessor coupled to the hardwareinstruction translator.

In yet another aspect, the present invention provides a computer programproduct encoded in at least one non-transitory computer usable mediumfor use with a computing device, the computer program product comprisingcomputer usable program code embodied in said medium for specifying amicroprocessor. The computer usable program code includes first programcode for specifying a plurality of registers that holds an architecturalstate of the microprocessor. The computer usable program code alsoincludes second program code for specifying an indicator that indicatesa boot instruction set architecture (ISA) of the microprocessor aseither the x86 ISA or the Advanced RISC Machines (ARM) ISA. The computerusable program code also includes third program code for specifying ahardware instruction translator that translates x86 ISA instructions andARM ISA instructions into microinstructions. The hardware instructiontranslator translates, as instructions of the boot ISA, the initial ISAinstructions that the microprocessor fetches from architectural memoryspace after receiving a reset signal. The computer usable program codealso includes fourth program code for specifying an execution pipeline,coupled to the hardware instruction translator. The execution pipelineexecutes the microinstructions to generate results defined by the x86ISA and ARM ISA instructions. In response to the reset signal, themicroprocessor initializes its architectural state in the plurality ofregisters as defined by the boot ISA prior to fetching the initial ISAinstructions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a microprocessor that runs x86ISA and ARM ISA machine language programs according to the presentinvention.

FIG. 2 is a block diagram illustrating in more detail the hardwareinstruction translator of FIG. 1.

FIG. 3 is a block diagram illustrating in more detail the instructionformatter of FIG. 2.

FIG. 4 is a block diagram illustrating in more detail the executionpipeline of FIG. 1.

FIG. 5 is a block diagram illustrating in more detail the register fileof FIG. 1.

FIGS. 6A and 6B are a flowchart illustrating operation of themicroprocessor of FIG. 1.

FIG. 7 is a block diagram illustrating a dual-core microprocessoraccording to the present invention.

FIG. 8 is a block diagram illustrating a microprocessor that runs x86ISA and ARM ISA machine language programs according to an alternateembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Glossary

An instruction set defines the mapping of a set of binary encodedvalues, which are machine language instructions, to operations themicroprocessor performs. (Typically, machine language programs areencoded in binary, although other number systems may be employed, forexample, the machine language programs of some older IBM computers wereencoded in decimal although they were ultimately represented bycollections of physical signals having voltages sensed as binaryvalues.) Illustrative examples of the types of operations machinelanguage instructions may instruct a microprocessor to perform are: addthe operand in register 1 to the operand in register 2 and write theresult to register 3, subtract the immediate operand specified in theinstruction from the operand in memory location 0x12345678 and write theresult to register 5, shift the value in register 6 by the number ofbits specified in register 7, branch to the instruction 36 bytes afterthis instruction if the zero flag is set, load the value from memorylocation 0xABCD0000 into register 8. Thus, the instruction set definesthe binary encoded value each machine language instruction must have tocause the microprocessor to perform the desired operation. It should beunderstood that the fact that the instruction set defines the mapping ofbinary values to microprocessor operations does not imply that a singlebinary value maps to a single microprocessor operation. Morespecifically, in some instruction sets, multiple binary values may mapto the same microprocessor operation.

An instruction set architecture (ISA), in the context of a family ofmicroprocessors, comprises: (1) an instruction set, (2) a set ofresources (e.g., registers and modes for addressing memory) accessibleby the instructions of the instruction set, and (3) a set of exceptionsthe microprocessor generates in response to processing the instructionsof the instruction set (e.g., divide by zero, page fault, memoryprotection violation). Because a programmer, such as an assembler orcompiler writer, who wants to generate a machine language program to runon a microprocessor family requires a definition of its ISA, themanufacturer of the microprocessor family typically defines the ISA in aprogrammer's manual. For example, at the time of its publication, theIntel 64 and IA-32 Architectures Software Developer's Manual, March 2009(consisting of five volumes, namely Volume 1: Basic Architecture; Volume2A: Instruction Set Reference, A-M; Volume 2B: Instruction SetReference, N-Z; Volume 3A: System Programming Guide; and Volume 3B:System Programming Guide, Part 2), which is hereby incorporated byreference herein in its entirety for all purposes, defined the ISA ofthe Intel 64 and IA-32 processor architecture, which is commonlyreferred to as the x86 architecture and which is also referred to hereinas x86, x86 ISA, x86 ISA family, x86 family or similar terms. Foranother example, at the time of its publication, the ARM ArchitectureReference Manual, ARM v7-A and ARM v7-R edition Errata markup, 2010,which is hereby incorporated by reference herein in its entirety for allpurposes, defined the ISA of the ARM processor architecture, which isalso referred to herein as ARM, ARM ISA, ARM ISA family, ARM family orsimilar terms. Other examples of well-known ISA families are IBMSystem/360/370/390 and z/Architecture, DEC VAX, Motorola 68k, MIPS,SPARC, PowerPC, and DEC Alpha. The ISA definition covers a family ofprocessors because over the life of the ISA processor family themanufacturer may enhance the ISA of the original processor in the familyby, for example, adding new instructions to the instruction set and/ornew registers to the architectural register set. To clarify by example,as the x86 ISA evolved it introduced in the Intel Pentium III processorfamily a set of 128-bit XMM registers as part of the SSE extensions, andx86 ISA machine language programs have been developed to utilize the XMMregisters to increase performance, although x86 ISA machine languageprograms exist that do not utilize the XMM registers of the SSEextensions. Furthermore, other manufacturers have designed andmanufactured microprocessors that run x86 ISA machine language programs.For example, Advanced Micro Devices (AMD) and VIA Technologies haveadded new features, such as the AMD 3DNOW! SIMD vector processinginstructions and the VIA Padlock Security Engine random number generatorand advanced cryptography engine features, each of which are utilized bysome x86 ISA machine language programs but which are not implemented incurrent Intel microprocessors. To clarify by another example, the ARMISA originally defined the ARM instruction set state, having 4-byteinstructions. However, the ARM ISA evolved to add, for example, theThumb instruction set state with 2-byte instructions to increase codedensity and the Jazelle instruction set state to accelerate Javabytecode programs, and ARM ISA machine language programs have beendeveloped to utilize some or all of the other ARM ISA instruction setstates, although ARM ISA machine language programs exist that do notutilize the other ARM ISA instruction set states.

A machine language program of an ISA comprises a sequence ofinstructions of the ISA, i.e., a sequence of binary encoded values thatthe ISA instruction set maps to the sequence of operations theprogrammer desires the program to perform. Thus, an x86 ISA machinelanguage program comprises a sequence of x86 ISA instructions; and anARM ISA machine language program comprises a sequence of ARM ISAinstructions. The machine language program instructions reside in memoryand are fetched and performed by the microprocessor.

A hardware instruction translator comprises an arrangement oftransistors that receives an ISA machine language instruction (e.g., anx86 ISA or ARM ISA machine language instruction) as input andresponsively outputs one or more microinstructions directly to anexecution pipeline of the microprocessor. The results of the executionof the one or more microinstructions by the execution pipeline are theresults defined by the ISA instruction. Thus, the collective executionof the one or more microinstructions by the execution pipeline“implements” the ISA instruction; that is, the collective execution bythe execution pipeline of the implementing microinstructions output bythe hardware instruction translator performs the operation specified bythe ISA instruction on inputs specified by the ISA instruction toproduce a result defined by the ISA instruction. Thus, the hardwareinstruction translator is said to “translate” the ISA instruction intothe one or more implementing microinstructions. The present disclosuredescribes embodiments of a microprocessor that includes a hardwareinstruction translator that translates x86 ISA instructions and ARM ISAinstructions into microinstructions. It should be understood that thehardware instruction translator is not necessarily capable oftranslating the entire set of instructions defined by the x86programmer's manual nor the ARM programmer's manual but rather iscapable of translating a subset of those instructions, just as the vastmajority of x86 ISA and ARM ISA processors support only a subset of theinstructions defined by their respective programmer's manuals. Morespecifically, the subset of instructions defined by the x86 programmer'smanual that the hardware instruction translator translates does notnecessarily correspond to any existing x86 ISA processor, and the subsetof instructions defined by the ARM programmer's manual that the hardwareinstruction translator translates does not necessarily correspond to anyexisting ARM ISA processor.

An execution pipeline is a sequence of stages in which each stageincludes hardware logic and a hardware register for holding the outputof the hardware logic for provision to the next stage in the sequencebased on a clock signal of the microprocessor. The execution pipelinemay include multiple such sequences of stages, i.e., multiple pipelines.The execution pipeline receives as input microinstructions andresponsively performs the operations specified by the microinstructionsto output results. The hardware logic of the various pipelines performsthe operations specified by the microinstructions that may include, butare not limited to, arithmetic, logical, memory load/store, compare,test, and branch resolution, and performs the operations on data informats that may include, but are not limited to, integer, floatingpoint, character, BCD, and packed. The execution pipeline executes themicroinstructions that implement an ISA instruction (e.g., x86 and ARM)to generate the result defined by the ISA instruction. The executionpipeline is distinct from the hardware instruction translator; morespecifically, the hardware instruction translator generates theimplementing microinstructions and the execution pipeline executes them;furthermore, the execution pipeline does not generate the implementingmicroinstructions.

An instruction cache is a random access memory device within amicroprocessor into which the microprocessor places instructions of anISA machine language program (such as x86 ISA and ARM ISA machinelanguage instructions) that were recently fetched from system memory andperformed by the microprocessor in the course of running the ISA machinelanguage program. More specifically, the ISA defines an instructionaddress register that holds the memory address of the next ISAinstruction to be performed (defined by the x86 ISA as an instructionpointer (IP) and by the ARM ISA as a program counter (PC), for example),and the microprocessor updates the instruction address register contentsas it runs the machine language program to control the flow of theprogram. The ISA instructions are cached for the purpose of subsequentlyfetching, based on the instruction address register contents, the ISAinstructions more quickly from the instruction cache rather than fromsystem memory the next time the flow of the machine language program issuch that the register holds the memory address of an ISA instructionpresent in the instruction cache. In particular, an instruction cache isaccessed based on the memory address held in the instruction addressregister (e.g., IP or PC), rather than exclusively based on a memoryaddress specified by a load or store instruction. Thus, a dedicated datacache that holds ISA instructions as data—such as may be present in thehardware portion of a system that employs a software translator—that isaccessed exclusively based on a load/store address but not by aninstruction address register value is not an instruction cache.Furthermore, a unified cache that caches both instructions and data,i.e., that is accessed based on an instruction address register valueand on a load/store address, but not exclusively based on a load/storeaddress, is intended to be included in the definition of an instructioncache for purposes of the present disclosure. In this context, a loadinstruction is an instruction that reads data from memory into themicroprocessor, and a store instruction is an instruction that writesdata to memory from the microprocessor.

A microinstruction set is the set of instructions (microinstructions)the execution pipeline of the microprocessor can execute.

DESCRIPTION OF THE EMBODIMENTS

The present disclosure describes embodiments of a microprocessor that iscapable of running both x86 ISA and ARM ISA machine language programs byhardware translating their respective x86 ISA and ARM ISA instructionsinto microinstructions that are directly executed by an executionpipeline of the microprocessor. The microinstructions are defined by amicroinstruction set of the microarchitecture of the microprocessordistinct from both the x86 ISA and the ARM ISA. As the microprocessorembodiments described herein run x86 and ARM machine language programs,a hardware instruction translator of the microprocessor translates thex86 and ARM instructions into the microinstructions and provides them tothe execution pipeline of the microprocessor that executes themicroinstructions that implement the x86 and ARM instructions.Advantageously, the microprocessor potentially runs the x86 and ARMmachine language programs faster than a system that employs a softwaretranslator since the implementing microinstructions are directlyprovided by the hardware instruction translator to the executionpipeline for execution, unlike a software translator-based system thatstores the host instructions to memory before they can be executed bythe execution pipeline.

Referring now to FIG. 1, a block diagram illustrating a microprocessor100 that can run x86 ISA and ARM ISA machine language programs accordingto the present invention is shown. The microprocessor 100 includes aninstruction cache 102; a hardware instruction translator 104 thatreceives x86 ISA instructions and ARM ISA instructions 124 from theinstruction cache 102 and translates them into microinstructions 126; anexecution pipeline 112 that receives the implementing microinstructions126 from the hardware instruction translator 104 executes them togenerate microinstruction results 128 that are forwarded back asoperands to the execution pipeline 112; a register file 106 and a memorysubsystem 108 that each provide operands to the execution pipeline 112and receive the microinstruction results 128 therefrom; an instructionfetch unit and branch predictor 114 that provides a fetch address 134 tothe instruction cache 102; an ARM ISA-defined program counter (PC)register 116 and an x86 ISA-defined instruction pointer (IP) register118 that are updated by the microinstruction results 128 and whosecontents are provided to the instruction fetch unit and branch predictor114; and configuration registers 122 that provide an instruction modeindicator 132 and an environment mode indicator 136 to the hardwareinstruction translator 104 and the instruction fetch unit and branchpredictor 114 and that are updated by the microinstruction results 128.

As the microprocessor 100 performs x86 ISA and ARM ISA machine languageinstructions, it fetches the instructions from system memory (not shown)into the microprocessor 100 according to the flow of the program. Themicroprocessor 100 caches the most recently fetched x86 ISA and ARM ISAmachine language instructions in the instruction cache 102. Theinstruction fetch unit 114 generates a fetch address 134 from which tofetch a block of x86 ISA or ARM ISA instruction bytes from systemmemory. The instruction cache 102 provides to the hardware instructiontranslator 104 the block of x86 ISA or ARM ISA instruction bytes 124 atthe fetch address 134 if it hits in the instruction cache 102;otherwise, the ISA instructions 124 are fetched from system memory. Theinstruction fetch unit 114 generates the fetch address 134 based on thevalues in the ARM PC 116 and x86 IP 118. More specifically, theinstruction fetch unit 114 maintains a fetch address in a fetch addressregister. Each time the instruction fetch unit 114 fetches a new blockof ISA instruction bytes, it updates the fetch address by the size ofthe block and continues sequentially in this fashion until a controlflow event occurs. The control flow events include the generation of anexception, the prediction by the branch predictor 114 that a takenbranch was present in the fetched block, and an update by the executionpipeline 112 to the ARM PC 116 and x86 IP 118 in response to a takenexecuted branch instruction that was not predicted taken by the branchpredictor 114. In response to a control flow event, the instructionfetch unit 114 updates the fetch address to the exception handleraddress, predicted target address, or executed target address,respectively. An embodiment is contemplated in which the instructioncache 102 is a unified cache in that it caches both ISA instructions 124and data. It is noted that in the unified cache embodiments, althoughthe unified cache may be accessed based on a load/store address toread/write data, when the microprocessor 100 fetches ISA instructions124 from the unified cache, the unified cache is accessed based on theARM PC 116 and x86 IP 118 values rather than a load/store address. Theinstruction cache 102 is a random access memory (RAM) device.

The instruction mode indicator 132 is state that indicates whether themicroprocessor 100 is currently fetching, formatting/decoding, andtranslating x86 ISA or ARM ISA instructions 124 into microinstructions126. Additionally, the execution pipeline 112 and memory subsystem 108receive the instruction mode indicator 132 which affects the manner ofexecuting the implementing microinstructions 126, albeit for arelatively small subset of the microinstruction set. The x86 IP register118 holds the memory address of the next x86 ISA instruction 124 to beperformed, and the ARM PC register 116 holds the memory address of thenext ARM ISA instruction 124 to be performed. To control the flow of theprogram, the microprocessor 100 updates the x86 IP register 118 and ARMPC register 116 as the microprocessor 100 performs the x86 and ARMmachine language programs, respectively, either to the next sequentialinstruction or to the target address of a branch instruction or to anexception handler address. As the microprocessor 100 performsinstructions of x86 ISA and ARM ISA machine language programs, itfetches the ISA instructions of the machine language programs fromsystem memory and places them into the instruction cache 102 replacingless recently fetched and performed instructions. The fetch unit 114generates the fetch address 134 based on the x86 IP register 118 or ARMPC register 116 value, depending upon whether the instruction modeindicator 132 indicates the microprocessor 100 is currently fetching ISAinstructions 124 in x86 or ARM mode. In one embodiment, the x86 IPregister 118 and the ARM PC register 116 are implemented as a sharedhardware instruction address register that provides its contents to theinstruction fetch unit and branch predictor 114 and that is updated bythe execution pipeline 112 according to x86 or ARM semantics based onwhether the instruction mode indicator 132 indicates x86 or ARM,respectively.

The environment mode indicator 136 is state that indicates whether themicroprocessor 100 is to apply x86 ISA or ARM ISA semantics to variousexecution environment aspects of the microprocessor 100 operation, suchas virtual memory, exceptions, cache control, and global execution-timeprotection. Thus, the instruction mode indicator 132 and environmentmode indicator 136 together create multiple modes of execution. In afirst mode in which the instruction mode indicator 132 and environmentmode indicator 136 both indicate x86 ISA, the microprocessor 100operates as a normal x86 ISA processor. In a second mode in which theinstruction mode indicator 132 and environment mode indicator 136 bothindicate ARM ISA, the microprocessor 100 operates as a normal ARM ISAprocessor. A third mode, in which the instruction mode indicator 132indicates x86 ISA but the environment mode indicator 136 indicates ARMISA, may advantageously be used to perform user mode x86 machinelanguage programs under the control of an ARM operating system orhypervisor, for example; conversely, a fourth mode, in which theinstruction mode indicator 132 indicates ARM ISA but the environmentmode indicator 136 indicates x86 ISA, may advantageously be used toperform user mode ARM machine language programs under the control of anx86 operating system or hypervisor, for example. The instruction modeindicator 132 and environment mode indicator 136 values are initiallydetermined at reset. In one embodiment, the initial values are encodedas microcode constants but may be modified by a blown configuration fuseand/or microcode patch. In another embodiment, the initial values areprovided by an external input to the microprocessor 100. In oneembodiment, the environment mode indicator 136 may only be changed afterreset by a reset-to-ARM 124 or reset-to-x86 instruction 124 (describedbelow with respect to FIG. 6); that is, the environment mode indicator136 may not be changed during normal operation of the microprocessor 100without resetting the microprocessor 100, either by a normal reset or bya reset-to-x86 or reset-to-ARM instruction 124.

The hardware instruction translator 104 receives as input the x86 ISAand ARM ISA machine language instructions 124 and in response to eachprovides as output one or more microinstructions 126 that implement thex86 or ARM ISA instruction 124. The collective execution of the one ormore implementing microinstructions 126 by the execution pipeline 112implements the x86 or ARM ISA instruction 124. That is, the collectiveexecution performs the operation specified by the x86 or ARM ISAinstruction 124 on inputs specified by the x86 or ARM ISA instruction124 to produce a result defined by the x86 or ARM ISA instruction 124.Thus, the hardware instruction translator 104 translates the x86 or ARMISA instruction 124 into the one or more implementing microinstructions126. The hardware instruction translator 104 comprises a collection oftransistors arranged in a predetermined manner to translate the x86 ISAand ARM ISA machine language instructions 124 into the implementingmicroinstructions 126. The hardware instruction translator 104 comprisesBoolean logic gates (e.g., of simple instruction translator 204 of FIG.2) that generate the implementing microinstructions 126. In oneembodiment, the hardware instruction translator 104 also comprises amicrocode ROM (e.g., element 234 of the complex instruction translator206 of FIG. 2) that the hardware instruction translator 104 employs togenerate implementing microinstructions 126 for complex ISA instructions124, as described in more detail with respect to FIG. 2. Preferably, thehardware instruction translator 104 is not necessarily capable oftranslating the entire set of ISA instructions 124 defined by the x86programmer's manual nor the ARM programmer's manual but rather iscapable of translating a subset of those instructions. Morespecifically, the subset of ISA instructions 124 defined by the x86programmer's manual that the hardware instruction translator 104translates does not necessarily correspond to any existing x86 ISAprocessor developed by Intel, and the subset of ISA instructions 124defined by the ARM programmer's manual that the hardware instructiontranslator 104 translates does not necessarily correspond to anyexisting ISA processor developed by ARM Ltd. The one or moreimplementing microinstructions 126 that implement an x86 or ARM ISAinstruction 124 may be provided to the execution pipeline 112 by thehardware instruction translator 104 all at once or as a sequence.Advantageously, the hardware instruction translator 104 provides theimplementing microinstructions 126 directly to the execution pipeline112 for execution without requiring them to be stored to memory inbetween. In the embodiment of the microprocessor 100 of FIG. 1, as themicroprocessor 100 runs an x86 or ARM machine language program, eachtime the microprocessor 100 performs an x86 or ARM instruction 124, thehardware instruction translator 104 translates the x86 or ARM machinelanguage instruction 124 into the implementing one or moremicroinstructions 126. However, the embodiment of FIG. 8 employs amicroinstruction cache to potentially avoid re-translation each time themicroprocessor 100 performs an x86 or ARM ISA instruction 124.Embodiments of the hardware instruction translator 104 are described inmore detail with respect to FIG. 2.

The execution pipeline 112 executes the implementing microinstructions126 provided by the hardware instruction translator 104. Broadlyspeaking, the execution pipeline 112 is a general purpose high-speedmicroinstruction processor, and other portions of the microprocessor100, such as the hardware instruction translator 104, perform the bulkof the x86/ARM-specific functions, although functions performed by theexecution pipeline 112 with x86/ARM-specific knowledge are discussedherein. In one embodiment, the execution pipeline 112 performs registerrenaming, superscalar issue, and out-of-order execution of theimplementing microinstructions 126 received from the hardwareinstruction translator 104. The execution pipeline 112 is described inmore detail with respect to FIG. 4.

The microarchitecture of the microprocessor 100 includes: (1) themicroinstruction set; (2) a set of resources accessible by themicroinstructions 126 of the microinstruction set, which is a supersetof the x86 ISA and ARM ISA resources; and (3) a set of micro-exceptionsthe microprocessor 100 is defined to generate in response to executingthe microinstructions 126, which is a superset of the x86 ISA and ARMISA exceptions. The microarchitecture is distinct from the x86 ISA andthe ARM ISA. More specifically, the microinstruction set is distinctfrom the x86 ISA and ARM ISA instruction sets in several aspects. First,there is not a one-to-one correspondence between the set of operationsthat the microinstructions of the microinstruction set may instruct theexecution pipeline 112 to perform and the set of operations that theinstructions of the x86 ISA and ARM ISA instruction sets may instructthe microprocessor to perform. Although many of the operations may bethe same, there may be some operations specifiable by themicroinstruction set that are not specifiable by the x86 ISA and/or theARM ISA instruction sets; conversely, there may be some operationsspecifiable by the x86 ISA and/or the ARM ISA instruction sets that arenot specifiable by the microinstruction set. Second, themicroinstructions of the microinstruction set are encoded in a distinctmanner from the manner in which the instructions of the x86 ISA and ARMISA instruction sets are encoded. That is, although many of the sameoperations (e.g., add, shift, load, return) are specifiable by both themicroinstruction set and the x86 ISA and ARM ISA instruction sets, thereis not a one-to-one correspondence between the binary opcodevalue-to-operation mappings of the microinstruction set and the x86 orARM ISA instruction sets. If there are binary opcode value-to-operationmappings that are the same in the microinstruction set and the x86 orARM ISA instruction set, they are, generally speaking, by coincidence,and there is nevertheless not a one-to-one correspondence between them.Third, the fields of the microinstructions of the microinstruction setdo not have a one-to-one correspondence with the fields of theinstructions of the x86 or ARM ISA instruction set.

The microprocessor 100, taken as a whole, can perform x86 ISA and ARMISA machine language program instructions. However, the executionpipeline 112 cannot execute x86 or ARM ISA machine language instructionsthemselves; rather, the execution pipeline 112 executes the implementingmicroinstructions 126 of the microinstruction set of themicroarchitecture of the microprocessor 100 into which the x86 ISA andARM ISA instructions are translated. However, although themicroarchitecture is distinct from the x86 ISA and the ARM ISA,alternate embodiments are contemplated in which the microinstruction setand other microarchitecture-specific resources are exposed to the user;that is, in the alternate embodiments the microarchitecture mayeffectively be a third ISA, in addition to the x86 ISA and ARM ISA,whose machine language programs the microprocessor 100 can perform.

Table 1 below describes some of the fields of a microinstruction 126 ofthe microinstruction set according to one embodiment of themicroprocessor 100.

TABLE 1 Field Description opcode operation to be performed (seeinstruction list below) destination specifies destination register ofmicroinstruction result source 1 specifies source of first input operand(e.g., general purpose register, floating point register,microarchitecture-specific register, condition flags register,immediate, displacement, useful constants, the next sequentialinstruction pointer value) source 2 specifies source of second inputoperand source 3 specifies source of third input operand (cannot be GPRor FPR) condition code condition upon which the operation will beperformed if satisfied and not performed if not satisfied operand sizeencoded number of bytes of operands used by this microinstructionaddress size encoded number of bytes of address generated by thismicroinstruction top of x87 FP needed for x87-style floating pointinstructions register stack

Table 2 below describes some of the microinstructions in themicroinstruction set according to one embodiment of the microprocessor100.

TABLE 2 Instruction Description ALU-type e.g., add, subtract, rotate,shift, Boolean, multiply, divide, floating-point ALU, media-type ALU(e.g., packed operations) load/store load from memory intoregister/store to memory from register conditional jump jump to targetaddress if condition is satisfied, e.g., zero, greater than, not equal;may specify either ISA flags or microarchitecture-specific (i.e.,non-ISA visible) condition flags move move value from source register todestination register conditional move move value from source register todestination register if condition is satisfied move to control movevalue from general purpose register to control register register movefrom move value to general purpose register from control controlregister register gprefetch guaranteed cache line prefetch instruction(i.e., not a hint, always prefetches, unless certain exceptionconditions) grabline performs zero beat read-invalidate cycle onprocessor bus to obtain exclusive ownership of cache line withoutreading data from system memory (since it is known the entire cache linewill be written) load pram load from PRAM (privatemicroarchitecture-specific RAM, i.e., not visible to ISA, described morebelow) into register store pram store to PRAM jump condition jump totarget address if “static” condition is satisfied on/off (withinrelevant timeframe, programmer guarantees there are no older, unretiredmicroinstructions that may change the “static” condition); fasterbecause resolved by complex instruction translator rather than executionpipeline call call subroutine return return from subroutine set biton/off set/clear bit in register copy bit copy bit value from sourceregister to destination register branch to next branch to nextsequential x86 or sequential ARM ISA instruction after the instructionx86 or ARM ISA instruction from which this pointer microinstruction wastranslated fence wait until all microinstructions have drained from theexecution pipeline to execute the microinstruction that comes after thismicroinstruction indirect jump unconditional jump through a registervalue

The microprocessor 100 also includes some microarchitecture-specificresources, such as microarchitecture-specific general purpose registers,media registers, and segment registers (e.g., used for register renamingor by microcode) and control registers that are not visible by the x86or ARM ISA, and a private RAM (PRAM) described more below. Additionally,the microarchitecture can generate exceptions, referred to asmicro-exceptions, that are not specified by and are not seen by the x86or ARM ISA, typically to perform a replay of a microinstruction 126 anddependent microinstructions 126, such as in the case of: a load miss inwhich the execution pipeline 112 assumes a load hit and replays the loadmicroinstruction 126 if it misses; a TLB miss, to replay themicroinstruction 126 after the page table walk and TLB fill; a floatingpoint microinstruction 126 that received a denormal operand that wasspeculated to be normal that needs to be replayed after the executionpipeline 112 normalizes the operand; a load microinstruction 126 thatwas executed, but after which an older address-colliding storemicroinstruction 126 was detected, requiring the load microinstruction126 to be replayed. It should be understood that the fields listed inTable 1, the microinstructions listed in Table 2, and themicroarchitecture-specific resources and microarchitecture-specificexceptions just listed are merely given as examples to illustrate themicroarchitecture and are by no means exhaustive.

The register file 106 includes hardware registers used by themicroinstructions 126 to hold source and/or destination operands. Theexecution pipeline 112 writes its results 128 to the register file 106and receives operands for the microinstructions 126 from the registerfile 106. The hardware registers instantiate the x86 ISA-defined and ARMISA-defined registers. In one embodiment, many of the general purposeregisters defined by the x86 ISA and the ARM ISA share some instances ofregisters of the register file 106. For example, in one embodiment, theregister file 106 instantiates fifteen 32-bit registers that are sharedby the ARM ISA registers R0 through R14 and the x86 ISA EAX through R14Dregisters. Thus, for example, if a first microinstruction 126 writes avalue to the ARM R2 register, then a subsequent second microinstruction126 that reads the x86 ECX register will receive the same value writtenby the first microinstruction 126, and vice versa. This advantageouslyenables x86 ISA and ARM ISA machine language programs to communicatequickly through registers. For example, assume an ARM machine languageprogram running under an ARM machine language operating system effects achange in the instruction mode 132 to x86 ISA and control transfer to anx86 machine language routine to perform a function, which may beadvantageous because the x86 ISA may support certain instructions thatcan perform a particular operation faster than in the ARM ISA. The ARMprogram can provide needed data to the x86 routine in shared registersof the register file 106. Conversely, the x86 routine can provide theresults in shared registers of the register file 106 that will bevisible to the ARM program upon return to it by the x86 routine.Similarly, an x86 machine language program running under an x86 machinelanguage operating system may effect a change in the instruction mode132 to ARM ISA and control transfer to an ARM machine language routine;the x86 program can provide needed data to the ARM routine in sharedregisters of the register file 106, and the ARM routine can provide theresults in shared registers of the register file 106 that will bevisible to the x86 program upon return to it by the ARM routine. Asixteenth 32-bit register that instantiates the x86 R15D register is notshared by the ARM R15 register since ARM R15 is the ARM PC register 116,which is separately instantiated. Additionally, in one embodiment, thethirty-two 32-bit ARM VFPv3 floating-point registers share 32-bitportions of the x86 sixteen 128-bit XMM0 through XMM15 registers and thesixteen 128-bit Advanced SIMD (“Neon”) registers. The register file 106also instantiates flag registers (namely the x86 EFLAGS register and ARMcondition flags register), and the various control and status registersdefined by the x86 ISA and ARM ISA. The architectural control and statusregisters include x86 architectural model specific registers (MSRs) andARM-reserved coprocessor (8-15) registers. The register file 106 alsoinstantiates non-architectural registers, such as non-architecturalgeneral purpose registers used in register renaming and used bymicrocode 234, as well as non-architectural x86 MSRs andimplementation-defined, or vendor-specific, ARM coprocessor registers.The register file 106 is described further with respect to FIG. 5.

The memory subsystem 108 includes a cache memory hierarchy of cachememories (in one embodiment, a level-1 instruction cache 102, level-1data cache, and unified level-2 cache). The memory subsystem 108 alsoincludes various memory request queues, e.g., load, store, fill, snoop,write-combine buffer. The memory subsystem 108 also includes a memorymanagement unit (MMU) that includes translation lookaside buffers(TLBs), preferably separate instruction and data TLBs. The memorysubsystem 108 also includes a table walk engine for obtaining virtual tophysical address translations in response to a TLB miss. Although shownseparately in FIG. 1, the instruction cache 102 is logically part of thememory subsystem 108. The memory subsystem 108 is configured such thatthe x86 and ARM machine language programs share a common memory space,which advantageously enables x86 and ARM machine language programs tocommunicate easily through memory.

The memory subsystem 108 is aware of the instruction mode 132 andenvironment mode 136 which enables it to perform various operations inthe appropriate ISA context. For example, the memory subsystem 108performs certain memory access violation checks (e.g., limit violationchecks) based on whether the instruction mode indicator 132 indicatesx86 or ARM ISA. For another example, in response to a change of theenvironment mode indicator 136, the memory subsystem 108 flushes theTLBs; however, the memory subsystem 108 does not flush the TLBs inresponse to a change of the instruction mode indicator 132, therebyenabling better performance in the third and fourth modes describedabove in which one of the instruction mode indicator 132 and environmentmode indicator 136 indicates x86 and the other indicates ARM. Foranother example, in response to a TLB miss, the table walk engineperforms a page table walk to populate the TLB using either x86 pagetables or ARM page tables depending upon whether the environment modeindicator 136 indicates x86 ISA or ARM ISA. For another example, thememory subsystem 108 examines the architectural state of the appropriatex86 ISA control registers that affect the cache policies (e.g., CR0 CDand NW bits) if the state indicator 136 indicates x86 ISA and examinesthe architectural state of the appropriate ARM ISA control registers(e.g., SCTLR I and C bits) if the environment mode indicator 136indicates ARM ISA. For another example, the memory subsystem 108examines the architectural state of the appropriate x86 ISA controlregisters that affect the memory management (e.g., CR0 PG bit) if thestate indicator 136 indicates x86 ISA and examines the architecturalstate of the appropriate ARM ISA control registers (e.g., SCTLR M bit)if the environment mode indicator 136 indicates ARM ISA. For anotherexample, the memory subsystem 108 examines the architectural state ofthe appropriate x86 ISA control registers that affect the alignmentchecking (e.g., CR0 AM bit) if the state indicator 136 indicates x86 ISAand examines the architectural state of the appropriate ARM ISA controlregisters (e.g., SCTLR A bit) if the environment mode indicator 136indicates ARM ISA. For another example, the memory subsystem 108 (aswell as the hardware instruction translator 104 for privilegedinstructions) examines the architectural state of the appropriate x86ISA control registers that specify the current privilege level (CPL) ifthe state indicator 136 indicates x86 ISA and examines the architecturalstate of the appropriate ARM ISA control registers that indicate user orprivileged mode if the environment mode indicator 136 indicates ARM ISA.However, in one embodiment, the x86 ISA and ARM ISA share controlbits/registers of the microprocessor 100 that have analogous function,rather than the microprocessor 100 instantiating separate controlbits/registers for each ISA.

Although shown separately, the configuration registers 122 may beconsidered part of the register file 106. The configuration registers122 include a global configuration register that controls operation ofthe microprocessor 100 in various aspects regarding the x86 ISA and ARMISA, such as the ability to enable or disable various features. Theglobal configuration register may be used to disable the ability of themicroprocessor 100 to perform ARM ISA machine language programs, i.e.,to make the microprocessor 100 an x86-only microprocessor 100, includingdisabling other relevant ARM-specific capabilities such as thelaunch-x86 and reset-to-x86 instructions 124 and implementation-definedcoprocessor registers described herein. The global configurationregister may also be used to disable the ability of the microprocessor100 to perform x86 ISA machine language programs, i.e., to make themicroprocessor 100 an ARM-only microprocessor 100, and to disable otherrelevant capabilities such as the launch-ARM and reset-to-ARMinstructions 124 and new non-architectural MSRs described herein. In oneembodiment, the microprocessor 100 is manufactured initially withdefault configuration settings, such as hardcoded values in themicrocode 234, which the microcode 234 uses at initialization time toconfigure the microprocessor 100, namely to write the configurationregisters 122. However, some configuration registers 122 are set byhardware rather than by microcode 234. Furthermore, the microprocessor100 includes fuses, readable by the microcode 234, which may be blown tomodify the default configuration values. In one embodiment, microcode234 reads the fuses and performs an exclusive-OR operation with thedefault value and the fuse value and uses the result to write to theconfiguration registers 122. Still further, the modifying effect of thefuses may be reversed by a microcode 234 patch. The global configurationregister may also be used, assuming the microprocessor 100 is configuredto perform both x86 and ARM programs, to determine whether themicroprocessor 100 (or a particular core 100 in a multi-core part, asdescribed with respect to FIG. 7) will boot as an x86 or ARMmicroprocessor when reset, or in response to an x86-style INIT, asdescribed in more detail below with respect to FIG. 6. The globalconfiguration register also includes bits that provide initial defaultvalues for certain architectural control registers, for example, the ARMISA SCTLT and CPACR registers. In a multi-core embodiment, such asdescribed with respect to FIG. 7, there exists a single globalconfiguration register, although each core is individually configurable,for example, to boot as either an x86 or ARM core, i.e., with theinstruction mode indicator 132 and environment mode indicator 136 bothset to x86 or ARM, respectively; furthermore, the launch-ARM instruction126 and launch-x86 instruction 126 may be used to dynamically switchbetween the x86 and ARM instruction modes 132. In one embodiment, theglobal configuration register is readable via an x86 RDMSR instructionto a new non-architectural MSR and a portion of the control bits thereinare writeable via an x86 WRMSR instruction to the new non-architecturalMSR, and the global configuration register is readable via an ARMMRC/MRRC instruction to an ARM coprocessor register mapped to the newnon-architectural MSR and the portion of the control bits therein arewriteable via an ARM MCR/MCRR instruction to the ARM coprocessorregister mapped to the new non-architectural MSR.

The configuration registers 122 also include various control registersthat control operation of the microprocessor 100 in various aspects thatare non-x86/ARM-specific, also referred to herein as global controlregisters, non-ISA control registers, non-x86/ARM control registers,generic control registers, and similar terms. In one embodiment, thesecontrol registers are accessible via both x86 RDMSR/WRMSR instructionsto non-architectural MSRs and ARM MCR/MRC (or MCRR/MRRC) instructions tonew implementation-defined coprocessor registers. For example, themicroprocessor 100 includes non-x86/ARM-specific control registers thatdetermine fine-grained cache control, i.e., finer-grained than providedby the x86 ISA and ARM ISA control registers.

In one embodiment, the microprocessor 100 provides ARM ISA machinelanguage programs access to the x86 ISA MSRs via implementation-definedARM ISA coprocessor registers that are mapped directly to thecorresponding x86 MSRs. The MSR address is specified in the ARM ISA R1register. The data is read from or written to the ARM ISA registerspecified by the MRC/MRRC/MCR/MCRR instruction. In one embodiment, asubset of the MSRs are password protected, i.e., the instructionattempting to access the MSR must provide a password; in thisembodiment, the password is specified in the ARM R7:R6 registers. If theaccess would cause an x86 general protection fault, the microprocessor100 causes an ARM ISA UND exception. In one embodiment, ARM coprocessor4 (address: 0, 7, 15, 0) is used to access the corresponding x86 MSRs.

The microprocessor 100 also includes an interrupt controller (not shown)coupled to the execution pipeline 112. In one embodiment, the interruptcontroller is an x86-style advanced programmable interrupt controller(APIC) that maps x86 ISA interrupts into ARM ISA interrupts. In oneembodiment, the x86 INTR maps to an ARM IRQ Interrupt; the x86 NMI mapsto an ARM IRQ Interrupt; the x86 INIT causes an INIT-reset sequence fromwhich the microprocessor 100 started in whichever ISA (x86 or ARM) itoriginally started out of a hardware reset; the x86 SMI maps to an ARMFIQ Interrupt; and the x86 STPCLK, A20, Thermal, PREQ, and Rebranch arenot mapped to ARM interrupts. ARM machine language programs are enabledto access the APIC functions via new implementation-defined ARMcoprocessor registers. In one embodiment, the APIC register address isspecified in the ARM R0 register, and the APIC register addresses arethe same as the x86 addresses. In one embodiment, ARM coprocessor 6(address: 0, 7, nn, 0, where nn is 15 for accessing the APIC, and 12-14for accessing the bus interface unit to perform 8-bit, 16-bit, and32-bit IN/OUT cycles on the processor bus) is used for privileged modefunctions typically employed by operating systems. The microprocessor100 also includes a bus interface unit (not shown), coupled to thememory subsystem 108 and execution pipeline 112, for interfacing themicroprocessor 100 to a processor bus. In one embodiment, the processorbus is conformant with one of the various Intel Pentium familymicroprocessor buses. ARM machine language programs are enabled toaccess the bus interface unit functions via new implementation-definedARM coprocessor registers in order to generate I/O cycles on theprocessor bus, i.e., IN and OUT bus transfers to a specified address inI/O space, which are needed to communicate with a chipset of a system,e.g., to generate an SMI acknowledgement special cycle, or I/O cyclesassociated with C-state transitions. In one embodiment, the I/O addressis specified in the ARM R0 register. In one embodiment, themicroprocessor 100 also includes power management capabilities, such asthe well-known P-state and C-state management. ARM machine languageprograms are enabled to perform power management via newimplementation-defined ARM coprocessor registers. In one embodiment, themicroprocessor 100 also includes an encryption unit (not shown) in theexecution pipeline 112. In one embodiment, the encryption unit issubstantially similar to the encryption unit of VIA microprocessors thatinclude the Padlock capability. ARM machine language programs areenabled to access the encryption unit functions, such as encryptioninstructions, via new implementation-defined ARM coprocessor registers.In one embodiment ARM coprocessor 5 is used for user mode functionstypically employed by user mode application programs, such as those thatmay use the encryption unit feature.

As the microprocessor 100 runs x86 ISA and ARM ISA machine languageprograms, the hardware instruction translator 104 performs the hardwaretranslation each time the microprocessor 100 performs an x86 or ARM ISAinstruction 124. It is noted that, in contrast, a softwaretranslator-based system may be able to improve its performance byre-using a translation in many cases rather than re-translating apreviously translated machine language instruction. Furthermore, theembodiment of FIG. 8 employs a microinstruction cache to potentiallyavoid re-translation each time the microprocessor 100 performs an x86 orARM ISA instruction 124. Each approach may have performance advantagesdepending upon the program characteristics and the particularcircumstances in which the program is run.

The branch predictor 114 caches history information about previouslyperformed both x86 and ARM branch instructions. The branch predictor 114predicts the presence and target address of both x86 and ARM branchinstructions 124 within a cache line as it is fetched from theinstruction cache 102 based on the cached history. In one embodiment,the cached history includes the memory address of the branch instruction124, the branch target address, a direction (taken/not taken) indicator,type of branch instruction, start byte within the cache line of thebranch instruction, and an indicator of whether the instruction wrapsacross multiple cache lines. In one embodiment, the branch predictor 114is enhanced to predict the direction of ARM ISA conditional non-branchinstructions, as described in U.S. Provisional Application No.61/473,067, filed Apr. 7, 2011, entitled APPARATUS AND METHOD FOR USINGBRANCH PREDICTION TO EFFICIENTLY EXECUTE CONDITIONAL NON-BRANCHINSTRUCTIONS. In one embodiment, the hardware instruction translator 104also includes a static branch predictor that predicts a direction andbranch target address for both x86 and ARM branch instructions based onthe opcode, condition code type, backward/forward, and so forth.

Various embodiments are contemplated that implement differentcombinations of features defined by the x86 ISA and ARM ISA. Forexample, in one embodiment, the microprocessor 100 implements the ARM,Thumb, ThumbEE, and Jazelle instruction set states, but provides atrivial implementation of the Jazelle extension; and implements thefollowing instruction set extensions: Thumb-2, VFPv3-D32, Advanced SIMD(“Neon”), multiprocessing, and VMSA; and does not implement thefollowing extensions: security extensions, fast context switchextension, ARM debug features (however, x86 debug functions areaccessible by ARM programs via ARM MCR/MRC instructions to newimplementation-defined coprocessor registers), performance monitoringcounters (however, x86 performance counters are accessible by ARMprograms via the new implementation-defined coprocessor registers). Foranother example, in one embodiment, the microprocessor 100 treats theARM SETEND instruction as a NOP and only supports the Little-endian dataformat. For another example, in one embodiment, the microprocessor 100does not implement the x86 SSE 4.2 capabilities.

Embodiments are contemplated in which the microprocessor 100 is anenhancement of a commercially available microprocessor, namely a VIANano™ Processor manufactured by VIA Technologies, Inc., of Taipei,Taiwan, which is capable of running x86 ISA machine language programsbut not ARM ISA machine language programs. The Nano microprocessorincludes a high performance register-renaming, superscalar instructionissue, out-of-order execution pipeline and a hardware translator thattranslates x86 ISA instructions into microinstructions for execution bythe execution pipeline. The Nano hardware instruction translator may besubstantially enhanced as described herein to translate ARM ISA machinelanguage instructions, in addition to x86 machine language instructions,into the microinstructions executable by the execution pipeline. Theenhancements to the hardware instruction translator may includeenhancements to both the simple instruction translator and to thecomplex instruction translator, including the microcode. Additionally,new microinstructions may be added to the microinstruction set tosupport the translation of ARM ISA machine language instructions intothe microinstructions, and the execution pipeline may be enhanced toexecute the new microinstructions. Furthermore, the Nano register fileand memory subsystem may be substantially enhanced as described hereinto support the ARM ISA, including sharing of certain registers. Thebranch prediction units may also be enhanced as described herein toaccommodate ARM branch instruction prediction in addition to x86branches. Advantageously, a relatively modest amount of modification isrequired to the execution pipeline of the Nano microprocessor toaccommodate the ARM ISA instructions since it is already largelyISA-agnostic. Enhancements to the execution pipeline may include themanner in which condition code flags are generated and used, thesemantics used to update and report the instruction pointer register,the access privilege protection method, and various memorymanagement-related functions, such as access violation checks, pagingand TLB use, and cache policies, which are listed only as illustrativeexamples, and some of which are described more below. Finally, asmentioned above, various features defined in the x86 ISA and ARM ISA maynot be supported in the Nano-enhancement embodiments, such as x86 SSE4.2 and ARM security extensions, fast context switch extension, debug,and performance counter features, which are listed only as illustrativeexamples, and some of which are described more below. The enhancement ofthe Nano processor to support running ARM ISA machine language programsis an example of an embodiment that makes synergistic use of design,testing, and manufacturing resources to potentially bring to market in atimely fashion a single integrated circuit design that can run both x86and ARM machine language programs, which represent the vast majority ofexisting machine language programs. In particular, embodiments of themicroprocessor 100 design described herein may be configured as an x86microprocessor, an ARM microprocessor, or a microprocessor that canconcurrently run both x86 ISA and ARM ISA machine language programs. Theability to concurrently run both x86 ISA and ARM ISA machine languageprograms may be achieved through dynamic switching between the x86 andARM instruction modes 132 on a single microprocessor 100 (or core100—see FIG. 7), through configuring one or more cores 100 in amulti-core microprocessor 100 (as described with respect to FIG. 7) asan ARM core and one or more cores as an x86 core, or through acombination of the two, i.e., dynamic switching between the x86 and ARMinstruction modes 132 on each of the multiple cores 100. Furthermore,historically, ARM ISA cores have been designed as intellectual propertycores to be incorporated into applications by various third-partyvendors, such as SOC and/or embedded applications. Therefore, the ARMISA does not specify a standardized processor bus to interface the ARMcore to the rest of the system, such as a chipset or other peripheraldevices. Advantageously, the Nano processor already includes a highspeed x86-style processor bus interface to memory and peripherals and amemory coherency structure that may be employed synergistically by themicroprocessor 100 to support running ARM ISA machine language programsin an x86 PC-style system environment.

Referring now to FIG. 2, a block diagram illustrating in more detail thehardware instruction translator 104 of FIG. 1 is shown. The hardwareinstruction translator 104 comprises hardware, more specifically acollection of transistors. The hardware instruction translator 104includes an instruction formatter 202 that receives the instruction modeindicator 132 and the blocks of x86 ISA and ARM ISA instruction bytes124 from the instruction cache 102 of FIG. 1 and outputs formatted x86ISA and ARM ISA instructions 242; a simple instruction translator (SIT)204 that receives the instruction mode indicator 132 and environmentmode indicator 136 and outputs implementing microinstructions 244 and amicrocode address 252; a complex instruction translator (CIT) 206 (alsoreferred to as a microcode unit) that receives the microcode address 252and the environment mode indicator 136 and provides implementingmicroinstructions 246; and a mux 212 that receives microinstructions 244from the simple instruction translator 204 on one input and thatreceives the microinstructions 246 from the complex instructiontranslator 206 on the other input and that provides the implementingmicroinstructions 126 to the execution pipeline 112 of FIG. 1. Theinstruction formatter 202 is described in more detail with respect toFIG. 3. The simple instruction translator 204 includes an x86 SIT 222and an ARM SIT 224. The complex instruction translator 206 includes amicro-program counter (micro-PC) 232 that receives the microcode address252, a microcode read only memory (ROM) 234 that receives a ROM address254 from the micro-PC 232, a microsequencer 236 that updates themicro-PC 232, an instruction indirection register (IIR) 235, and amicrotranslator 237 that generates the implementing microinstructions246 output by the complex instruction translator 206. Both theimplementing microinstructions 244 generated by the simple instructiontranslator 204 and the implementing microinstructions 246 generated bythe complex instruction translator 206 are microinstructions 126 of themicroinstruction set of the microarchitecture of the microprocessor 100and which are directly executable by the execution pipeline 112.

The mux 212 is controlled by a select input 248. Normally, the mux 212selects the microinstructions from the simple instruction translator204; however, when the simple instruction translator 204 encounters acomplex x86 or ARM ISA instruction 242 and transfers control, or traps,to the complex instruction translator 206, the simple instructiontranslator 204 controls the select input 248 to cause the mux 212 toselect microinstructions 246 from the complex instruction translator206. When the RAT 402 (of FIG. 4) encounters a microinstruction 126 witha special bit set to indicate it is the last microinstruction 126 in thesequence implementing the complex ISA instruction 242, the RAT 402controls the select input 248 to cause the mux 212 to return toselecting microinstructions 244 from the simple instruction translator204. Additionally, the reorder buffer 422 controls the select input 248to cause the mux 212 to select microinstructions 246 from the complexinstruction translator 206 when the reorder buffer 422 (see FIG. 4) isready to retire a microinstruction 126 whose status requires such, forexample if the status indicates the microinstruction 126 has caused anexception condition.

The simple instruction translator 204 receives the ISA instructions 242and decodes them as x86 ISA instructions if the instruction modeindicator 132 indicate x86 and decodes them as ARM ISA instructions ifthe instruction mode indicator 132 indicates ARM. The simple instructiontranslator 204 also determines whether the ISA instructions 242 aresimple or complex ISA instructions. A simple ISA instruction 242 is onefor which the simple instruction translator 204 can emit all theimplementing microinstructions 126 that implement the ISA instruction242; that is, the complex instruction translator 206 does not provideany of the implementing microinstructions 126 for a simple ISAinstruction 124. In contrast, a complex ISA instruction 124 requires thecomplex instruction translator 206 to provide at least some, if not all,of the implementing microinstructions 126. In one embodiment, for asubset of the instructions 124 of the ARM and x86 ISA instruction sets,the simple instruction translator 204 emits a portion of themicroinstructions 244 that implement the x86/ARM ISA instruction 126 andthen transfers control to the complex instruction translator 206 whichsubsequently emits the remainder of the microinstructions 246 thatimplement the x86/ARM ISA instruction 126. The mux 212 is controlled tofirst provide the implementing microinstructions 244 from the simpleinstruction translator 204 as microinstructions 126 to the executionpipeline 112 and second to provide the implementing microinstructions246 from the complex instruction translator 206 as microinstructions 126to the execution pipeline 112. The simple instruction translator 204knows the starting microcode ROM 234 address of the various microcoderoutines employed by the hardware instruction translator 104 to generatethe implementing microinstructions 126 for various complex ISAinstructions 124, and when the simple instruction translator 204 decodesa complex ISA instruction 242, it provides the relevant microcoderoutine address 252 to the micro-PC 232 of the complex instructiontranslator 206. The simple instruction translator 204 emits all themicroinstructions 244 needed to implement a relatively large percentageof the instructions 124 of the ARM and x86 ISA instruction sets,particularly ISA instructions 124 that tend to be performed by x86 ISAand ARM ISA machine language programs with a high frequency, and only arelatively small percentage requires the complex instruction translator206 to provide implementing microinstructions 246. According to oneembodiment, examples of x86 instructions that are primarily implementedby the complex instruction translator 206 are the RDMSR/WRMSR, CPUID,complex mathematical instructions (e.g., FSQRT and transcendentalinstructions), and IRET instructions; and examples of ARM instructionsthat are primarily implemented by the complex instruction translator 206are the MCR, MRC, MSR, MRS, SRS, and RFE instructions. The precedinglist is by no means exhaustive, but provides an indication of the typeof ISA instructions implemented by the complex instruction translator206.

When the instruction mode indicator 132 indicates x86, the x86 SIT 222decodes the x86 ISA instructions 242 and translates them into theimplementing microinstructions 244; when the instruction mode indicator132 indicates ARM, the ARM SIT 224 decodes the ARM ISA instructions 242and translates them into the implementing microinstructions 244. In oneembodiment, the simple instruction translator 204 is a block of Booleanlogic gates synthesized using well-known synthesis tools. In oneembodiment, the x86 SIT 222 and the ARM SIT 224 are separate blocks ofBoolean logic gates; however, in another embodiment, the x86 SIT 222 andthe ARM SIT 224 are a single block of Boolean logic gates. In oneembodiment, the simple instruction translator 204 translates up to threeISA instructions 242 and provides up to six implementingmicroinstructions 244 to the execution pipeline 112 per clock cycle. Inone embodiment, the simple instruction translator 204 comprises threesub-translators (not shown) that each translate a single formatted ISAinstruction 242: the first sub-translator is capable of translating aformatted ISA instruction 242 that requires no more than threeimplementing microinstructions 126; the second sub-translator is capableof translating a formatted ISA instruction 242 that requires no morethan two implementing microinstructions 126; and the thirdsub-translator is capable of translating a formatted ISA instruction 242that requires no more than one implementing microinstruction 126. In oneembodiment, the simple instruction translator 204 includes a hardwarestate machine that enables it to output multiple microinstructions 244that implement an ISA instruction 242 over multiple clock cycles.

In one embodiment, the simple instruction translator 204 also performsvarious exception checks based on the instruction mode indicator 132and/or environment mode indicator 136. For example, if the instructionmode indicator 132 indicates x86 and the x86 SIT 222 decodes an ISAinstruction 124 that is invalid for the x86 ISA, then the simpleinstruction translator 204 generates an x86 invalid opcode exception;similarly, if the instruction mode indicator 132 indicates ARM and theARM SIT 224 decodes an ISA instruction 124 that is invalid for the ARMISA, then the simple instruction translator 204 generates an ARMundefined instruction exception. For another example, if the environmentmode indicator 136 indicates the x86 ISA, then the simple instructiontranslator 204 checks to see whether each x86 ISA instruction 242 itencounters requires a particular privilege level and, if so, checkswhether the CPL satisfies the required privilege level for the x86 ISAinstruction 242 and generates an exception if not; similarly, if theenvironment mode indicator 136 indicates the ARM ISA, then the simpleinstruction translator 204 checks to see whether each formatted ARM ISAinstruction 242 is a privileged mode instruction and, if so, checkswhether the current mode is a privileged mode and generates an exceptionif the current mode is user mode. The complex instruction translator 206performs a similar function for certain complex ISA instructions 242.

The complex instruction translator 206 outputs a sequence ofimplementing microinstructions 246 to the mux 212. The microcode ROM 234stores ROM instructions 247 of microcode routines. The microcode ROM 234outputs the ROM instructions 247 in response to the address of the nextROM instruction 247 to be fetched from the microcode ROM 234, which isheld by the micro-PC 232. Typically, the micro-PC 232 receives itsinitial value 252 from the simple instruction translator 204 in responseto the simple instruction translator 204 decoding a complex ISAinstruction 242. In other cases, such as in response to a reset orexception, the micro-PC 232 receives the address of the reset microcoderoutine address or appropriate microcode exception handler address,respectively. The microsequencer 236 updates the micro-PC 232 normallyby the size of a ROM instruction 247 to sequence through microcoderoutines and alternatively to a target address generated by theexecution pipeline 112 in response to execution of a control typemicroinstruction 126, such as a branch instruction, to effect branchesto non-sequential locations in the microcode ROM 234. The microcode ROM234 is manufactured within the semiconductor die of the microprocessor100.

In addition to the microinstructions 244 that implement a simple ISAinstruction 124 or a portion of a complex ISA instruction 124, thesimple instruction translator 204 also generates ISA instructioninformation 255 that is written to the instruction indirection register(IIR) 235. The ISA instruction information 255 stored in the IIR 235includes information about the ISA instruction 124 being translated, forexample, information identifying the source and destination registersspecified by the ISA instruction 124 and the form of the ISA instruction124, such as whether the ISA instruction 124 operates on an operand inmemory or in an architectural register 106 of the microprocessor 100.This enables the microcode routines to be generic, i.e., without havingto have a different microcode routine for each different source and/ordestination architectural register 106. In particular, the simpleinstruction translator 204 is knowledgeable of the register file 106,including which registers are shared registers 504, and translates theregister information provided in the x86 ISA and ARM ISA instructions124 to the appropriate register in the register file 106 via the ISAinstruction information 255. The ISA instruction information 255 alsoincludes a displacement field, an immediate field, a constant field,rename information for each source operand as well as formicroinstruction 126 itself, information to indicate the first and lastmicroinstruction 126 in the sequence of microinstructions 126 thatimplement the ISA instruction 124, and other bits of useful informationgleaned from the decode of the ISA instruction 124 by the hardwareinstruction translator 104.

The microtranslator 237 receives the ROM instructions 247 from themicrocode ROM 234 and the contents of the IIR 235. In response, themicrotranslator 237 generates implementing microinstructions 246. Themicrotranslator 237 translates certain ROM instructions 247 intodifferent sequences of microinstructions 246 depending upon theinformation received from the IIR 235, such as depending upon the formof the ISA instruction 124 and the source and/or destinationarchitectural register 106 combinations specified by them. In manycases, much of the ISA instruction information 255 is merged with theROM instruction 247 to generate the implementing microinstructions 246.In one embodiment, each ROM instruction 247 is approximately 40 bitswide and each microinstruction 246 is approximately 200 bits wide. Inone embodiment, the microtranslator 237 is capable of generating up tothree microinstructions 246 from a ROM instruction 247. Themicrotranslator 237 comprises Boolean logic gates that generate theimplementing microinstructions 246.

An advantage provided by the microtranslator 237 is that the size of themicrocode ROM 234 may be reduced since it does not need to store the ISAinstruction information 255 provided by the IIR 235 since the simpleinstruction translator 204 generates the ISA instruction information255. Furthermore, the microcode ROM 234 routines may include fewerconditional branch instructions because it does not need to include aseparate routine for each different ISA instruction form and for eachsource and/or destination architectural register 106 combination. Forexample, if the complex ISA instruction 124 is a memory form, the simpleinstruction translator 204 may generate a prolog of microinstructions244 that includes microinstructions 244 to load the source operand frommemory into a temporary register 106, and the microtranslator 237 maygenerate a microinstruction 246 to store the result from the temporaryregister to memory; whereas, if the complex ISA instruction 124 is aregister form, the prolog may move the source operand from the sourceregister specified by the ISA instruction 124 to the temporary register106, and the microtranslator 237 may generate a microinstruction 246 tomove the result from a temporary register to the architecturaldestination register 106 specified by the IIR 235. In one embodiment,the microtranslator 237 is similar in many respects to themicrotranslator 237 described in U.S. patent application Ser. No.12/766,244, filed on Apr. 23, 2010, which is hereby incorporated byreference in its entirety for all purposes, but which is modified totranslate ARM ISA instructions 124 in addition to x86 ISA instructions124.

It is noted that the micro-PC 232 is distinct from the ARM PC 116 andthe x86 IP 118; that is, the micro-PC 232 does not hold the address ofISA instructions 124, and the addresses held in the micro-PC 232 are notwithin the system memory address space. It is further noted that themicroinstructions 246 are produced by the hardware instructiontranslator 104 and provided directly to the execution pipeline 112 forexecution rather than being results 128 of the execution pipeline 112.

Referring now to FIG. 3, a block diagram illustrating in more detail theinstruction formatter 202 of FIG. 2 is shown. The instruction formatter202 receives a block of the x86 ISA and ARM ISA instruction bytes 124from the instruction cache 102 of FIG. 1. By virtue of the variablelength nature of x86 ISA instructions, an x86 instruction 124 may beginin any byte within a block of instruction bytes 124. The task ofdetermining the length and location of an x86 ISA instruction within acache block is further complicated by the fact that the x86 ISA allowsprefix bytes and the length may be affected by current address lengthand operand length default values. Furthermore, ARM ISA instructions areeither 2-byte or 4-byte length instructions and are 2-byte or 4-bytealigned, depending upon the current ARM instruction set state 322 andthe opcode of the ARM ISA instruction 124. Therefore, the instructionformatter 202 extracts distinct x86 ISA and ARM ISA instructions fromthe stream of instruction bytes 124 made up of the blocks received fromthe instruction cache 102. That is, the instruction formatter 202formats the stream of x86 ISA and ARM ISA instruction bytes, whichgreatly simplifies the already difficult task of the simple instructiontranslator 204 of FIG. 2 to decode and translate the ISA instructions124.

The instruction formatter 202 includes a pre-decoder 302 thatpre-decodes the instruction bytes 124 as x86 instruction bytes if theinstruction mode indicator 132 indicates x86 and pre-decodes theinstruction bytes 124 as ARM instruction bytes if the instruction modeindicator 132 indicates ARM to generate pre-decode information. Aninstruction byte queue (IBQ) 304 receives the block of ISA instructionbytes 124 and associated pre-decode information generated by thepre-decoder 302.

An array of length decoders and ripple logic 306 receives the contentsof the bottom entry of the IBQ 304, namely a block of ISA instructionbytes 124 and associated pre-decode information. The length decoders andripple logic 306 also receives the instruction mode indicator 132 andthe ARM ISA instruction set state 322. In one embodiment, the ARM ISAinstruction set state 322 comprises the J and T bits of the ARM ISA CPSRregister. In response to its inputs, the length decoders and ripplelogic 306 generates decode information including the length of x86 andARM instructions in the block of ISA instruction bytes 124, x86 prefixinformation, and indicators associated with each of the ISA instructionbytes 124 indicating whether the byte is the start byte of an ISAinstruction 124, the end byte of an ISA instruction 124, and/or a validbyte of an ISA instruction 124. A mux queue (MQ) 308 receives a block ofthe ISA instruction bytes 126, its associated pre-decode informationgenerated by the pre-decoder 302, and the associated decode informationgenerated by the length decoders and ripple logic 306.

Control logic (not shown) examines the contents of the bottom MQ 308entries and controls muxes 312 to extract distinct, or formatted, ISAinstructions and associated pre-decode and decode information, which areprovided to a formatted instruction queue (FIQ) 314. The FIQ 314 buffersthe formatted ISA instructions 242 and related information for provisionto the simple instruction translator 204 of FIG. 2. In one embodiment,the muxes 312 extract up to three formatted ISA instructions and relatedinformation per clock cycle.

In one embodiment, the instruction formatter 202 is similar in many waysto the XIBQ, instruction formatter, and FIQ collectively as described inU.S. patent application Ser. Nos. 12/571,997; 12/572,002; 12/572,045;12/572,024; 12/572,052; 12/572,058, each filed on Oct. 1, 2009, whichare hereby incorporated by reference herein for all purposes. However,the XIBQ, instruction formatter, and FIQ of the above patentapplications are modified to format ARM ISA instructions 124 in additionto x86 ISA instructions 124. The length decoder 306 is modified todecode ARM ISA instructions 124 to generate their length and start, end,and valid byte indicators. In particular, if the instruction modeindicator 132 indicates ARM ISA, the length decoder 306 examines thecurrent ARM instruction set state 322 and the opcode of the ARM ISAinstruction 124 to determine whether the ARM instruction 124 is a 2-byteor 4-byte length instruction. In one embodiment, the length decoder 306includes separate length decoders for generating the length of x86 ISAinstructions 124 and for generating the length of ARM ISA instructions124, and tri-state outputs of the separate length decoders are wire-ORedtogether for provision to the ripple logic 306. In one embodiment, theformatted instruction queue (FIQ) 314 comprises separate queues forholding separate portions of the formatted instructions 242. In oneembodiment, the instruction formatter 202 provides the simpleinstruction translator 204 up to three formatted ISA instructions 242per clock cycle.

Referring now to FIG. 4, a block diagram illustrating in more detail theexecution pipeline 112 of FIG. 1 is shown. The execution pipeline 112 iscoupled to receive the implementing microinstructions 126 directly fromthe hardware instruction translator 104 of FIG. 2. The executionpipeline 112 includes a microinstruction queue 401 that receives themicroinstructions 126; a register allocation table (RAT) 402 thatreceives the microinstructions from the microinstruction queue 401; aninstruction dispatcher 404 coupled to the RAT 402; reservation stations406 coupled to the instruction dispatcher 404; an instruction issue unit408 coupled to the reservation stations 406; a reorder buffer (ROB) 422coupled to the RAT 402, instruction dispatcher 404, and reservationstations 406, and execution units 424 coupled to the reservationstations 406, instruction issue unit 408, and ROB 422. The RAT 402 andexecution units 424 receive the instruction mode indicator 132.

The microinstruction queue 401 operates as a buffer in circumstanceswhere the rate at which the hardware instruction translator 104generates the implementing microinstructions 126 differs from the rateat which the execution pipeline 112 executes them. In one embodiment,the microinstruction queue 401 comprises an M-to-N compressiblemicroinstruction queue that enables the execution pipeline 112 toreceive up to M (in one embodiment M is six) microinstructions 126 fromthe hardware instruction translator 104 in a given clock cycle and yetstore the received microinstructions 126 in an N-wide queue (in oneembodiment N is three) structure in order to provide up to Nmicroinstructions 126 per clock cycle to the RAT 402, which is capableof processing up to N microinstructions 126 per clock cycle. Themicroinstruction queue 401 is compressible in that it does not leaveholes among the entries of the queue, but instead sequentially fillsempty entries of the queue with the microinstructions 126 as they arereceived from the hardware instruction translator 104 regardless of theparticular clock cycles in which the microinstructions 126 are received.This advantageously enables high utilization of the execution units 424(of FIG. 4) in order to achieve high instruction throughput whileproviding advantages over a non-compressible M-wide or N-wideinstruction queue. More specifically, a non-compressible N-wide queuewould require the hardware instruction translator 104, in particular thesimple instruction translator 204, to re-translate in a subsequent clockcycle one or more ISA instructions 124 that it already translated in aprevious clock cycle because the non-compressible N-wide queue could notreceive more than N microinstructions 126 per clock cycle, and there-translation wastes power; whereas, a non-compressible M-wide queue,although not requiring the simple instruction translator 204 tore-translate, would create holes among the queue entries, which iswasteful and would require more rows of entries and thus a larger andmore power-consuming queue in order to accomplish comparable bufferingcapability.

The RAT 402 receives the microinstructions 126 from the microinstructionqueue 401 and generates dependency information regarding the pendingmicroinstructions 126 within the microprocessor 100 and performsregister renaming to increase the microinstruction parallelism to takeadvantage of the superscalar, out-of-order execution ability of theexecution pipeline 112. If the ISA instructions 124 indicates x86, thenthe RAT 402 generates the dependency information and performs theregister renaming with respect to the x86 ISA registers 106 of themicroprocessor 100; whereas, if the ISA instructions 124 indicates ARM,then the RAT 402 generates the dependency information and performs theregister renaming with respect to the ARM ISA registers 106 of themicroprocessor 100; however, as mentioned above, some of the registers106 may be shared by the x86 ISA and ARM ISA. The RAT 402 also allocatesan entry in the ROB 422 for each microinstruction 126 in program orderso that the ROB 422 can retire the microinstructions 126 and theirassociated x86 ISA and ARM ISA instructions 124 in program order, eventhough the microinstructions 126 may execute out of program order withrespect to the x86 ISA and ARM ISA instructions 124 they implement. TheROB 422 comprises a circular queue of entries, each for storinginformation related to a pending microinstruction 126. The informationincludes, among other things, microinstruction 126 execution status, atag that identifies the x86 or ARM ISA instruction 124 from which themicroinstruction 126 was translated, and storage for storing the resultsof the microinstruction 126.

The instruction dispatcher 404 receives the register-renamedmicroinstructions 126 and dependency information from the RAT 402 and,based on the type of instruction and availability of the execution units424, dispatches the microinstructions 126 and their associateddependency information to the reservation station 406 associated withthe appropriate execution unit 424 that will execute themicroinstruction 126.

The instruction issue unit 408, for each microinstruction 126 waiting ina reservation station 406, detects that the associated execution unit424 is available and the dependencies are satisfied (e.g., the sourceoperands are available) and issues the microinstruction 126 to theexecution unit 424 for execution. As mentioned, the instruction issueunit 408 can issue the microinstructions 126 for execution out ofprogram order and in a superscalar fashion.

In one embodiment, the execution units 424 include integer/branch units412, media units 414, load/store units 416, and floating point units418. The execution units 424 execute the microinstructions 126 togenerate results 128 that are provided to the ROB 422. Although theexecution units 424 are largely agnostic of whether themicroinstructions 126 they are executing were translated from an x86 orARM ISA instruction 124, the execution units 424 use the instructionmode indicator 132 and environment mode indicator 136 to execute arelatively small subset of the microinstructions 126. For example, theexecution pipeline 112 handles the generation of flags slightlydifferently based on whether the instruction mode indicator 132indicates the x86 ISA or the ARM ISA and updates the x86 EFLAGS registeror ARM condition code flags in the PSR depending upon whether theinstruction mode indicator 132 indicates the x86 ISA or the ARM ISA. Foranother example, the execution pipeline 112 samples the instruction modeindicator 132 to decide whether to update the x86 IP 118 or the ARM PC116, or common instruction address register, and whether to use x86 orARM semantics to do so. Once a microinstruction 126 becomes the oldestcompleted microinstruction 126 in the microprocessor 100 (i.e., at thehead of the ROB 422 queue and having a completed status) and all othermicroinstructions 126 that implement the associated ISA instruction 124are complete, the ROB 422 retires the ISA instruction 124 and frees upthe entries associated with the implementing microinstructions 126. Inone embodiment, the microprocessor 100 can retire up to three ISAinstructions 124 per clock cycle. Advantageously, the execution pipeline112 is a high performance, general purpose execution engine thatexecutes microinstructions 126 of the microarchitecture of themicroprocessor 100 that supports both x86 ISA and ARM ISA instructions124.

Referring now to FIG. 5, a block diagram illustrating in more detail theregister file 106 of FIG. 1 is shown. Preferably register file 106 isimplemented as separate physical blocks of registers. In one embodiment,the general purpose registers are implemented in one physical registerfile having a plurality of read ports and write ports; whereas, otherregisters may be physically located apart from the general purposeregister file and proximate functional blocks which access them and mayhave fewer read/write ports than the general purpose register file. Inone embodiment, some of the non-general purpose registers, particularlythose that do not directly control hardware of the microprocessor 100but simply store values used by microcode 234 (e.g., some x86 MSR or ARMcoprocessor registers), are implemented in a private random accessmemory (PRAM) accessible by the microcode 234 but invisible to the x86ISA and ARM ISA programmer, i.e., not within the ISA system memoryaddress space.

Broadly speaking, the register file 106 is separated logically intothree categories, as shown in FIG. 5, namely the ARM-specific registers502, the x86-specific register 504, and the shared registers 506. In oneembodiment, the shared registers 506 include fifteen 32-bit registersthat are shared by the ARM ISA registers R0 through R14 and the x86 ISAEAX through R14D registers as well as sixteen 128-bit registers sharedby the x86 ISA XMM0 through XMM15 registers and the ARM ISA AdvancedSIMD (Neon) registers, a portion of which are also overlapped by thethirty-two 32-bit ARM VFPv3 floating-point registers. As mentioned abovewith respect to FIG. 1, the sharing of the general purpose registersimplies that a value written to a shared register by an x86 ISAinstruction 124 will be seen by an ARM ISA instruction 124 thatsubsequently reads the shared register, and vice versa. Thisadvantageously enables x86 ISA and ARM ISA routines to communicate withone another through registers. Additionally, as mentioned above, certainbits of architectural control registers of the x86 ISA and ARM ISA arealso instantiated as shared registers 506. As mentioned above, in oneembodiment, the x86 MSRs may be accessed by ARM ISA instructions 124 viaan implementation-defined coprocessor register, and are thus shared bythe x86 ISA and ARM ISA. The shared registers 506 may also includenon-architectural registers, for example non-architectural equivalentsof the condition flags, that are also renamed by the RAT 402. Thehardware instruction translator 104 is aware of which registers areshared by the x86 ISA and ARM ISA so that it may generate theimplementing microinstructions 126 that access the correct registers.

The ARM-specific registers 502 include the other registers defined bythe ARM ISA that are not included in the shared registers 506, and thex86-specific registers 504 include the other registers defined by thex86 ISA that are not included in the shared registers 506. Examples ofthe ARM-specific registers 502 include the ARM PC 116, CPSR, SCTRL,FPSCR, CPACR, coprocessor registers, banked general purpose registersand SPSRs of the various exception modes, and so forth. The foregoing isnot intended as an exhaustive list of the ARM-specific registers 502,but is merely provided as an illustrative example. Examples of thex86-specific registers 504 include the x86 EIP 118, EFLAGS, R15D, upper32 bits of the 64-bit R0-R15 registers (i.e., the portion not in theshared registers 506), segment registers (SS, CS, DS, ES, FS, GS), x87FPU registers, MMX registers, control registers (e.g., CR0-CR3, CR8),and so forth. The foregoing is not intended as an exhaustive list of thex86-specific registers 504, but is merely provided as an illustrativeexample.

In one embodiment, the microprocessor 100 includes newimplementation-defined ARM coprocessor registers that may be accessedwhen the instruction mode indicator 132 indicates the ARM ISA in orderto perform x86 ISA-related operations, including but not limited to: theability to reset the microprocessor 100 to an x86 ISA processor(reset-to-x86 instruction); the ability to initialize the x86-specificstate of the microprocessor 100, switch the instruction mode indicator132 to x86, and begin fetching x86 instructions 124 at a specified x86target address (launch-x86 instruction); the ability to access theglobal configuration register discussed above; the ability to accessx86-specific registers (e.g., EFLAGS), in which the x86 register to beaccessed is identified in the ARM R0 register, power management (e.g.,P-state and C-state transitions), processor bus functions (e.g., I/Ocycles), interrupt controller access, and encryption accelerationfunctionality access, as discussed above. Furthermore, in oneembodiment, the microprocessor 100 includes new x86 non-architecturalMSRs that may be accessed when the instruction mode indicator 132indicates the x86 ISA in order to perform ARM ISA-related operations,including but not limited to: the ability to reset the microprocessor100 to an ARM ISA processor (reset-to-ARM instruction); the ability toinitialize the ARM-specific state of the microprocessor 100, switch theinstruction mode indicator 132 to ARM, and begin fetching ARMinstructions 124 at a specified ARM target address (launch-ARMinstruction); the ability to access the global configuration registerdiscussed above; the ability to access ARM-specific registers (e.g., theCPSR), in which the ARM register to be accessed is identified in the EAXregister.

Referring now to FIG. 6, comprising FIGS. 6A and 6B, a flowchartillustrating operation of the microprocessor 100 of FIG. 1 is shown.Flow begins at block 602.

At block 602, the microprocessor 100 is reset. The reset may be signaledon the reset input to the microprocessor 100. Additionally, in anembodiment in which the processor bus is an x86 style processor bus, thereset may be signaled by an x86-style INIT. In response to the reset,the reset routines in the microcode 234 are invoked. The resetmicrocode: (1) initializes the x86-specific state 504 to the defaultvalues specified by the x86 ISA; (2) initializes the ARM-specific state502 to the default values specified by the ARM ISA; (3) initializes thenon-ISA-specific state of the microprocessor 100 to the default valuesspecified by the microprocessor 100 manufacturer; (4) initializes theshared ISA state 506, e.g., the GPRs, to the default values specified bythe x86 ISA; and (5) sets the instruction mode indicator 132 andenvironment mode indicator 136 to indicate the x86 ISA. In an alternateembodiment, instead of actions (4) and (5) above, the reset microcodeinitializes the shared ISA state 506 to the default values specified bythe ARM ISA and sets the instruction mode indicator 132 and environmentmode indicator 136 to indicate the ARM ISA. In such an embodiment, theactions at blocks 638 and 642 would not need to be performed, and beforeblock 614 the reset microcode would initialize the shared ISA state 506to the default values specified by the x86 ISA and set the instructionmode indicator 132 and environment mode indicator 136 to indicate thex86 ISA. Flow proceeds to block 604.

At block 604, the reset microcode determines whether the microprocessor100 is configured to boot as an x86 processor or as an ARM processor. Inone embodiment, as described above, the default ISA boot mode ishardcoded in microcode but may be modified by blowing a configurationfuse and/or by a microcode patch. In another embodiment, the default ISAboot mode is provided as an external input to the microprocessor 100,such as an external input pin. Flow proceeds to decision block 606. Atdecision block 606, if the default ISA boot mode is x86, flow proceedsto block 614; whereas, if the default ISA boot mode is ARM, flowproceeds to block 638.

At block 614, the reset microcode causes the microprocessor 100 to beginfetching x86 instructions 124 at the reset vector address specified bythe x86 ISA. Flow proceeds to block 616.

At block 616, the x86 system software, e.g., BIOS, configures themicroprocessor 100 using, for example, x86 ISA RDMSR and WRMSRinstructions 124. Flow proceeds to block 618.

At block 618, the x86 system software does a reset-to-ARM instruction124. The reset-to-ARM instruction causes the microprocessor 100 to resetand to come out of the reset as an ARM processor. However, because nox86-specific state 504 and no non-ISA-specific configuration state ischanged by the reset-to-ARM instruction 126, it advantageously enablesx86 system firmware to perform the initial configuration of themicroprocessor 100 and then reboot the microprocessor 100 as an ARMprocessor while keeping intact the non-ARM configuration of themicroprocessor 100 performed by the x86 system software. This enables“thin” micro-boot code to boot an ARM operating system without requiringthe micro-boot code to know the complexities of how to configure themicroprocessor 100. In one embodiment, the reset-to-ARM instruction isan x86 WRMSR instruction to a new non-architectural MSR. Flow proceedsto block 622.

At block 622, the simple instruction translator 204 traps to the resetmicrocode in response to the complex reset-to-ARM instruction 124. Thereset microcode initializes the ARM-specific state 502 to the defaultvalues specified by the ARM ISA. However, the reset microcode does notmodify the non-ISA-specific state of the microprocessor 100, whichadvantageously preserves the configuration performed at block 616.Additionally, the reset microcode initializes the shared ISA state 506to the default values specified by the ARM ISA. Finally, the resetmicrocode sets the instruction mode indicator 132 and environment modeindicator 136 to indicate the ARM ISA. Flow proceeds to block 624.

At block 624, the reset microcode causes the microprocessor 100 to beginfetching ARM instructions 124 at the address specified in the x86 ISAEDX:EAX registers. Flow ends at block 624.

At block 638, the reset microcode initializes the shared ISA state 506,e.g., the GPRs, to the default values specified by the ARM ISA. Flowproceeds to block 642.

At block 642, the reset microcode sets the instruction mode indicator132 and environment mode indicator 136 to indicate the ARM ISA. Flowproceeds to block 644.

At block 644, the reset microcode causes the microprocessor 100 to beginfetching ARM instructions 124 at the reset vector address specified bythe ARM ISA. The ARM ISA defines two reset vector addresses selected byan input. In one embodiment, the microprocessor 100 includes an externalinput to select between the two ARM ISA-defined reset vector addresses.In another embodiment, the microcode 234 includes a default selectionbetween the two ARM ISA-defined reset vector addresses, which may bemodified by a blown fuse and/or microcode patch. Flow proceeds to block646.

At block 646, the ARM system software configures the microprocessor 100using, for example, ARM ISA MCR and MRC instructions 124. Flow proceedsto block 648.

At block 648, the ARM system software does a reset-to-x86 instruction124. The reset-to-x86 instruction causes the microprocessor 100 to resetand to come out of the reset as an x86 processor. However, because noARM-specific state 502 and no non-ISA-specific configuration state ischanged by the reset-to-x86 instruction 126, it advantageously enablesARM system firmware to perform the initial configuration of themicroprocessor 100 and then reboot the microprocessor 100 as an x86processor while keeping intact the non-x86 configuration of themicroprocessor 100 performed by the ARM system software. This enables“thin” micro-boot code to boot an x86 operating system without requiringthe micro-boot code to know the complexities of how to configure themicroprocessor 100. In one embodiment, the reset-to-x86 instruction isan ARM MRC/MRCC instruction to a new implementation-defined coprocessorregister. Flow proceeds to block 652.

At block 652, the simple instruction translator 204 traps to the resetmicrocode in response to the complex reset-to-x86 instruction 124. Thereset microcode initializes the x86-specific state 504 to the defaultvalues specified by the x86 ISA. However, the reset microcode does notmodify the non-ISA-specific state of the microprocessor 100, whichadvantageously preserves the configuration performed at block 646.Additionally, the reset microcode initializes the shared ISA state 506to the default values specified by the x86 ISA. Finally, the resetmicrocode sets the instruction mode indicator 132 and environment modeindicator 136 to indicate the x86 ISA. Flow proceeds to block 654.

At block 654, the reset microcode causes the microprocessor 100 to beginfetching x86 instructions 124 at the address specified in the ARM ISAR1:R0 registers. Flow ends at block 654.

Referring now to FIG. 7, a block diagram illustrating a dual-coremicroprocessor 700 according to the present invention is shown. Thedual-core microprocessor 700 includes two processing cores 100 in whicheach core 100 includes the elements of the microprocessor 100 of FIG. 1such that it can perform both x86 ISA and ARM ISA machine languageprograms. The cores 100 may be configured such that both cores 100 arerunning x86 ISA programs, both cores 100 are running ARM ISA programs,or one core 100 is running x86 ISA programs while the other core 100 isrunning ARM ISA programs, and the mix between these three configurationsmay change dynamically during operation of the microprocessor 700. Asdiscussed above with respect to FIG. 6, each core 100 has a defaultvalue for its instruction mode indicator 132 and environment modeindicator 136, which may be inverted by a fuse and/or microcode patch,such that each core 100 may individually come out of reset as an x86 oran ARM processor. Although the embodiment of FIG. 7 includes two cores100, in other embodiments the microprocessor 700 includes more than twocores 100, each capable of running both x86 ISA and ARM ISA machinelanguage programs.

Referring now to FIG. 8, a block diagram illustrating a microprocessor100 that can perform x86 ISA and ARM ISA machine language programsaccording to an alternate embodiment of the present invention is shown.The microprocessor 100 of FIG. 8 is similar to the microprocessor 100 ofFIG. 1 and like-numbered elements are similar. However, themicroprocessor 100 of FIG. 8 also includes a microinstruction cache 892.The microinstruction cache 892 caches microinstructions 126 generated bythe hardware instruction translator 104 that are provided directly tothe execution pipeline 112. The microinstruction cache 892 is indexed bythe fetch address 134 generated by the instruction fetch unit 114. Ifthe fetch address 134 hits in the microinstruction cache 892, then a mux(not shown) within the execution pipeline 112 selects themicroinstructions 126 from the microinstruction cache 892 rather thanfrom the hardware instruction translator 104; otherwise, the mux selectsthe microinstructions 126 provided directly from the hardwareinstruction translator 104. The operation of a microinstruction cache,also commonly referred to as a trace cache, is well-known in the art ofmicroprocessor design. An advantage provided by the microinstructioncache 892 is that the time required to fetch the microinstructions 126from the microinstruction cache 892 is typically less than the timerequired to fetch the ISA instructions 124 from the instruction cache102 and translate them into the microinstructions 126 by the hardwareinstruction translator 104. In the embodiment of FIG. 8, as themicroprocessor 100 runs an x86 or ARM ISA machine language program, thehardware instruction translator 104 may not need to perform the hardwaretranslation each time it performs an x86 or ARM ISA instruction 124,namely if the implementing microinstructions 126 are already present inthe microinstruction cache 892.

Advantageously, embodiments of a microprocessor are described hereinthat can run both x86 ISA and ARM ISA machine language programs byincluding a hardware instruction translator that translates both x86 ISAand ARM ISA instructions into microinstructions of a microinstructionset distinct from the x86 ISA and ARM ISA instruction sets, whichmicroinstructions are executable by a common execution pipeline of themicroprocessor to which the implementing microinstructions are provided.An advantage of embodiments of the microprocessor described herein isthat, by synergistically utilizing the largely ISA-agnostic executionpipeline to execute microinstructions that are hardware translated fromboth x86 ISA and ARM ISA instructions, the design and manufacture of themicroprocessor may require fewer resources than two separately designedand manufactured microprocessors, i.e., one that can perform x86 ISAmachine language programs and one that can perform ARM ISA machinelanguage programs. Additionally, embodiments of the microprocessor,particularly those which employ a superscalar out-of-order executionpipeline, potentially provide a higher performance ARM ISA processorthan currently exists. Furthermore, embodiments of the microprocessorpotentially provide higher x86 and ARM performance than a system thatemploys a software translator. Finally, the microprocessor may beincluded in a system on which both x86 and ARM machine language programscan be run concurrently with high performance due to its ability toconcurrently run both x86 ISA and ARM ISA machine language programs.

While various embodiments of the present invention have been describedherein, it should be understood that they have been presented by way ofexample, and not limitation. It will be apparent to persons skilled inthe relevant computer arts that various changes in form and detail canbe made therein without departing from the scope of the invention. Forexample, software can enable, for example, the function, fabrication,modeling, simulation, description and/or testing of the apparatus andmethods described herein. This can be accomplished through the use ofgeneral programming languages (e.g., C, C++), hardware descriptionlanguages (HDL) including Verilog HDL, VHDL, and so on, or otheravailable programs. Such software can be disposed in any known computerusable medium such as magnetic tape, semiconductor, magnetic disk, oroptical disc (e.g., CD-ROM, DVD-ROM, etc.), a network or othercommunications medium. Embodiments of the apparatus and method describedherein may be included in a semiconductor intellectual property core,such as a microprocessor core (e.g., embodied, or specified, in a HDL)and transformed to hardware in the production of integrated circuits.Additionally, the apparatus and methods described herein may be embodiedas a combination of hardware and software. Thus, the present inventionshould not be limited by any of the exemplary embodiments describedherein, but should be defined only in accordance with the followingclaims and their equivalents. Specifically, the present invention may beimplemented within a microprocessor device which may be used in ageneral purpose computer. Finally, those skilled in the art shouldappreciate that they can readily use the disclosed conception andspecific embodiments as a basis for designing or modifying otherstructures for carrying out the same purposes of the present inventionwithout departing from the scope of the invention as defined by theappended claims.

We claim:
 1. A microprocessor, comprising: a plurality of registers,that holds an architectural state of the microprocessor; an indicator,that indicates a boot instruction set architecture (ISA) of themicroprocessor as either the x86 ISA or the Advanced RISC Machines (ARM)ISA; a hardware instruction translator, that translates x86 ISAinstructions and ARM ISA instructions into microinstructions, whereinthe hardware instruction translator translates, as instructions of theboot ISA, the initial ISA instructions that the microprocessor fetchesfrom architectural memory space after receiving a reset signal; anexecution pipeline, coupled to the hardware instruction translator,wherein the execution pipeline executes the microinstructions togenerate results defined by the x86 ISA and ARM ISA instructions; andwherein in response to the reset signal, the microprocessor initializesits architectural state in the plurality of registers as defined by theboot ISA prior to fetching the initial ISA instructions.
 2. Themicroprocessor of claim 1, wherein the indicator comprises a defaultvalue in microcode of the microprocessor.
 3. The microprocessor of claim2, further comprising: a fuse, which may be blown to invert the defaultvalue in the microcode.
 4. The microprocessor of claim 2, wherein thedefault value in the microcode is modifiable by a microcode patchapplied prior to the microprocessor fetching its initial ISA instructionfrom architectural memory space after being reset.
 5. The microprocessorof claim 1, wherein the indicator comprises a bit in a register of themicroprocessor that has a default value in response to the reset signal.6. The microprocessor of claim 1, wherein the indicator comprises anexternal input to the microprocessor.
 7. The microprocessor of claim 1,wherein in response to the reset signal the microprocessor fetches theinitial ISA instructions from architectural memory space at an addressdefined by the boot ISA.
 8. A method, comprising: detecting that a resetof a microprocessor has been signaled, wherein said detecting isperformed by the microprocessor; determining, in response to saiddetecting that a reset of the microprocessor has been signaled, which ofthe x86 instruction set architecture (ISA) and the Advanced RISCMachines (ARM) ISA an indicator of the microprocessor indicates is aboot ISA of the microprocessor, wherein said determining is performed bythe microprocessor; initializing an architectural state of themicroprocessor as defined by the boot ISA, wherein said initializing isperformed by the microprocessor; translating into microinstructions theinitial ISA instructions fetched from architectural memory space by themicroprocessor after the reset as instructions of the boot ISA, whereinsaid translating is performed by a hardware instruction translator ofthe microprocessor; and executing the microinstructions to generateresults defined by the boot ISA, wherein said executing is performed byan execution pipeline of the microprocessor coupled to the hardwareinstruction translator.
 9. The method of claim 8, wherein the indicatorcomprises a default value in microcode of the microprocessor.
 10. Themethod of claim 9, further comprising: inverting the default value inthe microcode in response to sensing a blown fuse of the microprocessor.11. The method of claim 9, wherein the default value in the microcode ismodifiable by a microcode patch applied prior to the microprocessorfetching its initial ISA instruction from architectural memory spaceafter being reset.
 12. The method of claim 8, wherein the indicatorcomprises a bit in a register of the microprocessor that has a defaultvalue in response to the reset signal.
 13. The method of claim 8,wherein the indicator comprises an external input to the microprocessor.14. The method of claim 8, further comprising: fetching the initial ISAinstructions after the reset at an address defined by the boot ISA. 15.A computer program product encoded in at least one non-transitorycomputer usable medium for use with a computing device, the computerprogram product comprising: computer usable program code embodied insaid medium, for specifying a microprocessor, the computer usableprogram code comprising: first program code for specifying a pluralityof registers, that holds an architectural state of the microprocessor;second program code for specifying an indicator, that indicates a bootinstruction set architecture (ISA) of the microprocessor as either thex86 ISA or the Advanced RISC Machines (ARM) ISA; third program code forspecifying a hardware instruction translator, that translates x86 ISAinstructions and ARM ISA instructions into microinstructions, whereinthe hardware instruction translator translates, as instructions of theboot ISA, the initial ISA instructions that the microprocessor fetchesfrom architectural memory space after receiving a reset signal; fourthprogram code for specifying an execution pipeline, coupled to thehardware instruction translator, wherein the execution pipeline executesthe microinstructions to generate results defined by the x86 ISA and ARMISA instructions; and wherein in response to the reset signal, themicroprocessor initializes its architectural state in the plurality ofregisters as defined by the boot ISA prior to fetching the initial ISAinstructions.
 16. The computer program product of claim 15, wherein theindicator comprises a default value in microcode of the microprocessor.17. The computer program product of claim 16, further comprising: afuse, which may be blown to invert the default value in the microcode.18. The computer program product of claim 15, wherein the indicatorcomprises a bit in a register of the microprocessor that has a defaultvalue in response to the reset signal.
 19. The computer program productof claim 15, wherein the indicator comprises an external input to themicroprocessor.
 20. The computer program product of claim 15, wherein inresponse to the reset signal the microprocessor fetches the initial ISAinstructions from architectural memory space at an address defined bythe boot ISA.